If you can identify the actual shares you sold, you can use the adjusted basis of those shares to calculate your gain or loss.
In order to use the specific identification method, you must:
Tell the fund or your broker the specific shares that are to be sold or transferred at the time of the sale or transfer, and
Receive written confirmation of your specification from the fund or your broker within a reasonable time.
The potential advantage of this method can best be explained with an illustration. Say you bought 100 shares of a fund for $10 of a share in 2004, another 100 shares for $20 in 2005 and another 100 shares for $30 a share in mid-2006. And, in late 2006, you want to sell 100 shares for $25 a share. If you direct the fund to sell the shares purchased in 2006, you have $500 short-term loss; sell the shares purchased in 2004, though, and you have a $1,500 long-term gain. Either way, you'll get $2,500 from the redemption. Which choice is best for you depends on overall results of your trading for the year.