<prompt> Human: Dear PF,I am finally! creating a Vanguard account and setting up my Roth IRA. I noticed in the Vanguard account that some mutual funds mention they are also available as an ETF which tends to have lower expense ratios. I understand the basic difference between ETFs and Mutual funds. But given that the typical advise is to invest in Vanguard mutual funds, I was wondering what are the downsides to investing in the ETF version of a fund, rather than the Mutual Fund version? Are there hidden costs or complications in handling an ETF investment? Or are there no downsides and we should be investing in the lower cost ETF version if it is an option? Thanks for your guidance!  <response>: You cant buy fractional shares, and there is a bidask spread.For high-volume ETFs like the core offerings from Vanguard the bidask spread is minimal, and the lower expense ratios more than make up for having a few dollars uninvested, especially as your total balance grows. 