VOO and FXAIX both track the S&P 500, so inside a Roth IRA their returns are near-identical. FXAIX charges 0.015% versus VOO's 0.03%, a difference of about $1.50 a year per $10,000. The real decision is platform fit and how you want to buy: FXAIX for exact-dollar auto-investing, VOO for an ETF you can hold anywhere.
Key takeaways
- VOO is the Vanguard S&P 500 ETF (expense ratio 0.03%, per Vanguard as of June 30, 2026). FXAIX is the Fidelity 500 Index Fund, a mutual fund (expense ratio 0.015%, per Fidelity as of August 2026).
- Both hold the same 500 companies in the same weights, so gross returns are effectively identical. The only built-in performance gap is the 0.015 percentage point expense ratio difference.
- The usual "ETFs are more tax-efficient than mutual funds" argument does not apply here. A Roth IRA is already tax-free, so the ETF structure's tax advantage is worth nothing inside it.
- FXAIX lets you invest an exact dollar amount and buy fractional shares by default, which makes automatic recurring contributions clean. VOO needs a broker that supports fractional ETF shares to do the same.
- Pick on your platform. FXAIX is native to Fidelity. VOO is portable and works at any broker. Holding VOO at Fidelity is also fine.
VOO vs FXAIX at a glance
| Feature | VOO | FXAIX |
|---|---|---|
| Type | ETF (Vanguard) | Mutual fund (Fidelity) |
| Expense ratio | 0.03% | 0.015% |
| Minimum investment | Price of a share, or any amount where fractional ETF shares are supported | No minimum |
| Fractional / exact-dollar | Only if your broker supports fractional ETF shares | Yes, by default |
| Trading | Intraday, live price | Once daily at closing NAV |
| Best platform | Any broker (fully portable) | Fidelity (native) |
Why the usual ETF tax argument does not apply in a Roth
Outside a retirement account, the standard case for an ETF over a mutual fund is tax efficiency. ETFs use an in-kind creation and redemption process that lets them avoid passing capital gains distributions to shareholders. Traditional index mutual funds like FXAIX can occasionally distribute capital gains, which trigger a tax bill in a taxable brokerage account.
Inside a Roth IRA, none of that matters. A Roth is already tax-free: no tax on dividends, no tax on capital gains distributions, and no tax on qualified withdrawals in retirement. Whether a fund distributes capital gains or not has zero effect on what you keep, because the account shelters all of it.
So the single biggest reason people reach for VOO over a comparable mutual fund vanishes the moment you are inside a Roth. What is left is the plain stuff: the expense ratio, how you buy shares, and which brokerage you use. On that list FXAIX has the lower expense ratio (0.015% vs 0.03%), and its mutual-fund structure is actually an advantage for exact-dollar automatic investing.
How you actually buy them
This is where the two diverge in daily use.
FXAIX is a mutual fund, so you buy in dollars. Tell Fidelity to invest $500 and it puts the full $500 to work at that day's closing net asset value, fractional shares included. Set up a recurring $500-a-month contribution and every dollar goes in, no leftover cash. Mutual funds only price once per day at NAV, so you do not get to pick an intraday price, which for a long-term Roth holder is a non-issue.
VOO is an ETF, so it trades like a stock at a live price during market hours. To invest an exact dollar amount you need a broker that supports fractional ETF shares. Fidelity, Schwab, and Robinhood do; some others still make you buy whole shares, which leaves idle cash between contributions. If your broker supports fractional ETF trading, VOO behaves almost like FXAIX for set-it-and-forget-it investing. If it does not, automatic investing is clunkier.
Which fits which brokerage
FXAIX is a Fidelity fund. If your Roth IRA is at Fidelity, FXAIX is the natural pick: no transaction fee, no minimum, clean dollar-based automatic investing. Buying FXAIX at a non-Fidelity broker can be awkward or carry a transaction fee, so it is really a Fidelity play.
VOO is portable. As an ETF it trades commission-free at virtually every major broker, so it is the better choice if your Roth lives at Vanguard, Schwab, or anywhere that is not Fidelity. And holding VOO inside a Fidelity Roth is perfectly fine too, especially if you want fractional-share dollar investing on an ETF you could later move to another broker in kind. If you are building a Roth at Fidelity and want to see how VOO stacks up against Fidelity's own ETF lineup, compare the best Fidelity ETFs for a Roth IRA.
The verdict
For a Roth IRA, this is close to a coin flip, and you cannot make a bad choice. Both give you the same S&P 500 exposure at a rock-bottom cost, and the tax-free wrapper erases the ETF's usual tax edge.
Choose FXAIX if your Roth is at Fidelity and you want the lowest expense ratio (0.015%) with the cleanest dollar-based automatic investing. Choose VOO if your Roth is anywhere else, or if you want a portable ETF you can carry between brokers, and your platform supports fractional shares so recurring contributions stay tidy. The 0.015 percentage point expense ratio gap is real but tiny, about $1.50 a year per $10,000, so let platform and buying mechanics drive the decision, not the fee.
Want to see how a mutual fund index differs from a same-index ETF in a different matchup? Our breakdown of SPY vs the S&P 500 covers the ETF-tracking side, and if you are weighing a factor tilt against a plain index fund, compare the Vanguard S&P 500 Value ETF.
Frequently asked questions
Is there a real performance difference between VOO and FXAIX in a Roth IRA?
No meaningful performance difference exists, because both hold the same 500 companies in the same weights, so gross returns are effectively identical. The only built-in gap is the expense ratio: FXAIX charges 0.015% versus VOO's 0.03%, worth about $1.50 a year per $10,000. Let platform and buying mechanics drive the decision, not the fee.
Why does the ETF tax advantage not matter inside a Roth IRA?
The ETF tax advantage vanishes inside a Roth IRA because a Roth is already tax-free. Outside a retirement account, ETFs avoid passing capital gains distributions to shareholders while mutual funds can trigger a tax bill. But a Roth shelters all of it, with no tax on dividends, capital gains distributions, or qualified withdrawals, so the fund's structure has zero effect on what you keep.
Which is easier for automatic recurring contributions, VOO or FXAIX?
FXAIX is easier for automatic recurring contributions because it is a mutual fund you buy in exact dollar amounts with fractional shares by default. Tell Fidelity to invest $500 and the full $500 goes to work at that day's closing NAV, no leftover cash. VOO needs a broker that supports fractional ETF shares to do the same, otherwise idle cash sits between contributions.
Should I pick VOO or FXAIX based on my brokerage?
Yes, pick based on your brokerage. FXAIX is a Fidelity fund, so it is the natural choice with no transaction fee or minimum if your Roth is at Fidelity, but it can be awkward or carry a fee elsewhere. VOO is portable and trades commission-free at virtually every major broker, making it the better pick if your Roth lives at Vanguard, Schwab, or anywhere else.