The phlebotomine sand flies Lutzomyia (Nyssomyia) intermedia (Lutz & Neiva) and Lutzomyia (Nyssomyia) whitmani (Coutinho & Antunes) are important vectors of Leishmania (Vianna) braziliensis, the etiological agent of American cutaneous leishmaniasis. In some areas, both species occur in sympatry, and their relative roles as vectors in these areas are not clear. We studied the nocturnal activity and biting rhythms of both species in Posse, a locality in Rio de Janeiro State, Brazil. Our results show differences between the activity patterns of Lu. intermedia and Lu. whitmani that might be epidemiologically important. Although the activity profiles vary between seasons and microhabitats (peridomestic versus forest), the two species show marked differences in their tendencies to bite humans in the early morning (0400–0600 hours), with Lu. whitmani showing higher feeding rates than Lu. intermedia.