If you want the simplest, cheapest way to own the U.S. stock market inside a Fidelity account, the Fidelity 500 Index Fund (NASDAQ:FXAIX) is the default answer. FXAIX tracks the S&P 500 and has become one of the largest index mutual funds in the country because it solves a single problem cleanly: getting diversified large-cap U.S. equity exposure with almost no friction. The distinction worth understanding is that FXAIX is a mutual fund, not an ETF, and that shapes how it fits in a portfolio.
The Job FXAIX Is Built to Do
FXAIX exists to be a core holding. It replicates the S&P 500, meaning you own roughly proportional slices of the 500 largest U.S. companies, weighted by market cap. The return engine is simple: capital appreciation from those underlying companies, plus reinvested dividends from the index, minus a sliver for fund expenses. There are no options overlays, no factor tilts, no manager picking winners. The fund’s published expense ratio of 0.015% sits among the lowest in the industry, which means almost every dollar of index return flows through to shareholders.
For a Fidelity brokerage or 401(k) participant, FXAIX is often the path of least resistance to S&P 500 exposure. It trades at NAV once a day, has no bid/ask spread, and accepts dollar-based purchases down to the penny, which makes it well suited for automated payroll contributions and dividend reinvestment.
Does the Performance Match the Promise?
The strategy is only useful if the tracking is tight. By that test, FXAIX delivers. Over the past year, the fund returned about 30%, while SPDR S&P 500 ETF Trust (NYSEARCA:SPY | SPY Price Prediction) returned about 28% over the same window. Five-year total return for FXAIX came in around 86%, and the ten-year figure landed near 318%. The gap versus SPY’s 252% ten-year price return is largely explained by FXAIX’s adjusted figures including reinvested dividends, while SPY’s price-only series strips them out.
FXAIX has hugged the index almost perfectly, which is exactly what you want from a passive vehicle. The current backdrop reinforces the case. The 10-year Treasury sits near 4.45%, the Fed funds upper bound is at 3.75% after three cuts since September, and the VIX has settled around 17 after spiking above 31 in late March. Equity index funds tend to perform well when policy is loosening and volatility is normalizing.
The Tradeoffs Worth Naming
Three constraints deserve attention. First, concentration. The S&P 500 is now heavily weighted toward a handful of mega-cap technology names, so FXAIX is less diversified than its 500-stock label suggests. A drawdown in the largest holdings drags the whole fund.
Second, mutual fund mechanics. Because FXAIX trades only at the 4 p.m. NAV, you cannot sell intraday, set a limit price, or use it in a taxable brokerage with the same tax efficiency as an ETF. Index mutual funds occasionally distribute capital gains, while ETF rivals like Vanguard S&P 500 ETF (NYSEARCA:VOO) and iShares Core S&P 500 ETF (NYSEARCA:IVV), each at 0.03% expense ratios, almost never do.
Third, portability. FXAIX is a Fidelity proprietary fund, so transferring it to Schwab or Vanguard typically forces a sale, which can trigger taxes in a non-retirement account.
Who It Fits and Who Should Look Elsewhere
FXAIX makes sense as a core 30% to 70% holding for Fidelity account holders, particularly inside 401(k)s and IRAs where tax distributions are irrelevant and intraday trading does not matter. For a taxable account, or for any investor who values portability across brokerages, VOO or IVV deliver effectively identical exposure with ETF tax treatment at a fractionally higher expense ratio that is unlikely to be felt. Anyone seeking income, downside protection, or international diversification needs to pair FXAIX with something else. The fund does one job: it gives you the S&P 500 at near zero cost. Judged on that job, it is hard to fault.
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