Paul,

The solution to your immediate problem is to use the second argument for the STR() function which tells fox how many digits to use;

INDEX ON STR(AcctNo,3) + STR(OtherNum,2)

The more permanent solution is to change the data type of these fields as they are NOT numbers, but rather are strings of digits. The proof of this is this question, "When will you need to calculate the average or the sum of account number(s)?" If the answer is never, and it most likely is in this situation, then these fields are NOT numbers they a strings. Changing the data type will eliminate all of the str() stuff.