Promethea (Callosamia promethea) and tulip tree silk moths (C. angulifera) were compared under semi-natural conditions for the presence of polyandry. Promethea were polyandrous, the first documentation for a saturniid moth, whereas tulip tree moths were monandrous. Experiments showed that polyandrous and monandrous promethea females achieved similar egg fertility, but polyandrous females laid 10% more eggs, a significant difference (P < 0.05). This difference was found in five sibling groups, whose larvae were reared on the same individual food plant. Higher fecundity for polyandrous promethea females was not related to female size, duration of copulation, egg size and number, number of days in laying period or sibling group. Egg size declined later in the laying period for all females. Small females laid more eggs earlier in the laying period than large females. A separate mark/recapture study showed that male promethea also mate multiple times (polygyny) and distinguish virgin from nonvirgin females in mating preference. The diurnally mating promethea may gain increased genetic variability and/or possibly seminal gifts from polyandry. Monandry in totally nocturnal saturniid moths may result from a time conflict between egg laying and mating, which overrides the benefits of polyandry. If polyandry increases fecundity, it is predicted to occur in other diurnally mating saturniids.