maximum number of six letter words that the language can employ

An anthropologist discovers an isolated tribe whose written alphabet contains only six letters (call the letters A, B, C,D,E,and F).  The tribe has a taboo against using the same letter twice in the same word.  It is never done.  If each different sequence of letters constitures a different word in the language, what is the maiximum number of six-letter words that the language can employ?