Strategies of males in harem-forming mammals may change depending on 2 variables, female group size and consequent ability of the harem male to monopolize copulations, and the relative costs and benefits of tolerating other males. We studied harems of 4–20 females associated with a dominant male in small groups of Artibeus jamaicensis (<14 females), and with dominant and subordinate males in large groups (>14 females). Dominant males displayed defensive behavior toward satellite males when they intruded into the roosting site. Small groups received the highest number of visits by satellite males, and dominant males did not display total defense of females. During the breeding season, females roosted in highly compacted clusters and dominant males were more active in their defensive behavior. Subordinate males were generally tolerated in harem groups and their presence reduced the number of adult male visits. Some young in large harem groups were sired by subordinate males, resulting in a genetic benefit for both dominant and subordinate males. Dominant males had the highest fitness in the large harem groups by sharing paternity with related subordinate males and by rejecting unrelated intruders.