Across the past 20 years, captive reproduction has declined steeply in mongoose lemurs (Eulemur mongoz) and has been only moderately successful in black lemurs (E. macuco). At the same time, reproduction has been so successful in brown lemurs (E. fulvus) that contraception has been used since 1987 to limit captive numbers. No obvious cause for the differential reproductive success has been identified. Our experience with a pair of unrelated mongoose lemurs at the Philadelphia Zoological Garden suggested that they reproduced in only those years when they were caged in close proximity to another nonreproductive, fullsibling pair of mongoose lemurs. Examination of the worldwide pattern of captive reproduction by Eulemur species during the past two decades revealed that female black and mongoose lemurs housed in institutions with either additional conspecific males or additional conspecific pairs had a higher rate of reproduction than those maintained as an isolated pair. Black lemurs also had higher rates of reproduction when additional conspecific females were present. No similar pattern was found for brown lemurs or for two comparable mammals, the pudu (Pudu pudu) and the okapi (Okupia johnstoni). The evidence presented for the black and mongoose lemurs supports the existence of the Allee effect in these two species, namely, that reproduction is enhanced by the presence of conspecifics.