If your mutual fund splits, or adjusts its price, it is treated like a stock split. Your total basis doesn't change after the split, but since you now own more shares without paying any more money, your per-share basis will decrease. To calculate your per-share basis, divide the total cost that you have invested in the fund (minus any shares previously sold) by the current number of shares that you hold.

References:

• Publication 550,Investment Income and Expenses

Source: www.irs.gov