I am treating my HSA as a retirement account. To max it out I contribute about $100 every pay period (roughly every two weeks) pre-tax, plus employer contributions sprinkled in occasionally. Once I got to $2500 I bought one of Fidelity's no-fee mutual funds (FAMRX) since that was the minimum investment. Now I am wondering what to do with the small contributions that come in. Thus far I have been using them to buy more of the same mutual fund, since there does not seem to be any costs associated with doing this. But I am wary that this may be a bad idea for some reason. Should I instead let the contributions sit in cash until I have enough to make a decent-sized investment in something else?