Investing
Two ways to own a basket of investments, how they trade, what they cost, and where the TSP funds fit.
Paratroopers board a shared aircraft, JBER, May 11, 2021. U.S. Army photo, JBER, DVIDS (public domain).
A mutual fund and an ETF are both baskets of investments you can own in one purchase. The main difference is how and when you trade them. A mutual fund is a basket you buy from and sell back to the fund at one price set after the market closes each day. An ETF, short for exchange-traded fund, is a fund that trades on an exchange all day at market prices, the way a single stock does.
For a buy-and-hold investor, that timing gap matters less than it sounds. What matters more is cost, and the Thrift Savings Plan already hands you low-cost baskets at some of the cheapest prices around.
Both hold a basket of investments. What changes is how and when you buy and sell, and the fees that ride along.
Mutual fund
ETF
Either way, watch fees
For a buy-and-hold investor, the timing difference matters less than it sounds.
Source: SEC · Investor.gov
The big difference is how and when you buy and sell. A mutual fund pools investors' money into a portfolio, and you buy or sell shares directly with the fund at one price set once a day after the market closes. An ETF holds a basket too, but its shares trade on a stock exchange throughout the day at market prices, the way a single stock does.
So an ETF combines the pooled-basket feature of a mutual fund with the all-day trading of an exchange-listed fund. For a long-term investor, that timing gap matters less than it sounds. You are buying and holding, not day-trading.
Both charge an expense ratio, which is the yearly fee that covers running the fund. Index-style versions of either tend to cost less than actively managed ones, and fees can lower your returns over time. Small percentage differences add up to real money across a career.
There are trading costs to think about too. Because ETFs trade on an exchange, buying and selling them may involve a brokerage commission or a small gap between the buy and sell price, depending on your broker. Mutual funds skip the all-day trading but can carry their own charges, such as a sales load, which is a one-time sales charge on some funds. The cheapest path, either way, is a low-cost index fund held for the long haul.
Here is where the picture changes once you put on the uniform.
The TSP core funds work like index funds at rock-bottom cost. The mutual-fund-vs-ETF choice only comes up if you open an account on the outside.
0.034–0.051%
the TSP's 2025 expense ratios, lower than 99% of investment options, with no commissions or sales loads.
What this means for you
You do not have to choose ETF vs mutual fund to start.
Source: TSP.gov
The TSP's core funds, the G, F, C, S, I, and L funds, act like index funds: they aim to match a market index at very low cost. The difference is access. They live only inside your TSP account. You cannot buy or sell them on a public exchange the way you would an ETF, and there is no all-day price to chase. You set a contribution percentage or move money between funds, and the TSP handles it.
Where the TSP shines is cost. Its total expense ratios ran about 0.034% to 0.051% across the individual funds in 2025, and the TSP states its expenses are lower than 99% of investment options. There is no commission to buy or sell between TSP funds, and no sales loads. For a junior service member, that means the TSP is often cheaper than a comparable mutual fund or ETF you would buy on the outside.
So you do not have to choose ETF versus mutual fund to get started. The TSP already gives you index-style baskets at rock-bottom cost. If you later open an IRA on the outside, that is where the mutual fund versus ETF choice actually comes up, and even there, a low-cost index version of either can do the job.
What is the difference between a mutual fund and an ETF?
A mutual fund is bought from and sold to the fund at one price set after the market closes, while an ETF trades on an exchange throughout the day at market prices. Both hold a basket of investments.
Are ETFs cheaper than mutual funds?
It depends on the specific funds. Index versions of either are usually low-cost, but ETFs may involve a brokerage commission to trade, and some mutual funds carry sales loads. Higher fees can reduce returns over time.
Is the TSP a mutual fund?
Not exactly. The TSP funds work like low-cost index funds, but they are held only inside your TSP account and are not bought or sold on a public exchange the way a mutual fund or ETF is.
Can I buy an ETF inside my TSP?
Not directly through the core funds. The TSP does offer a mutual fund window for a wider menu, but it carries extra fees, so most junior investors stick with the low-cost core and L funds.
Which should I pick for an IRA, a mutual fund or an ETF?
Either can work for a long-term investor. The bigger factors are keeping fees low and staying diversified. Compare expense ratios and any trading costs before deciding.
Do ETFs and mutual funds give instant diversification?
It depends on the fund. A broad fund can spread you across many companies in one purchase, but a narrowly focused fund, one sector for example, does not on its own provide full diversification.
