If you go the the Harvard website (www.harvard.edu), click on Admission & Financial Aid, then click on the link for undergraduates. Once you're here you want to click on Admission & Financial Aid FAQ. This will give you a list of topics. Click on Standerdized Tests found under the admissions category. There you will find an answer to your question along with other similar questions. Hope this helps!
Along with the SAT I, Harvard requires three SAT II subject tests. Taking the ACT too won't hurt, though.