=(LEN(A1)-LEN(SUBSTITUTE(LOWER(A1),lower("a"),"")))/LEN("a")
Will count the number of A's or a's in A1.

If you want just the lower case a's:
=(LEN(A1)-LEN(SUBSTITUTE(A1,"a","")))/LEN("a")



Bob wrote:

For a given cell, I need to count the number of times a given character
(e.g., the letter "a") appears. Is there a worksheet function or formula
that will help me do this? Thanks for any help.

--

Dave Peterson