Data from wild and semifree-ranging groups of ringtailed lemurs (Lemur catta) were combined to evaluate the hypothesis that female ringtailed lemurs exhibit asynchrony of estrus within seasonal synchrony of estrous cycles. Photoperiodic and probably social entrainments lead all females within social groups of ringtailed lemurs to experience estrus annually within periods of 7 to 20 days. Among an average of five adult females in each of 11 group-years, however, only 2 separate instances of dyadic estrous overlap were observed. Computer simulations of estrus occurring independently among group-living females revealed that this was unlikely to have occurred by chance. Recent research on social enhancement and suppression of ovarian cycles in mammals suggests that a single pheromone-based signal-response system could mediate both ringtailed lemurs' remarkable annual estrous synchrony and the subsidiary asynchrony reported here. Asynchrony of estrus probably functions to maximize each female's ability to exercise mate choice by circumventing temporal conflict among females. Asynchrony of estrus and female mate choice cause current models to explain male membership in primate groups to fail for ringtailed lemurs. Such results highlight the need for detailed information on behavioral and physiological reproductive tactics before generally applicable models of reproductive strategies can be developed.