The sex ratio ofBemisia tabaciwas examined in the field, and ranged from a female-biased condition early in the season to male biased at summer's end. Laboratory studies showed that young females laid proportionately more female-producing eggs than older females, and that females with access to males throughout their life laid a higher proportion of females than those that mated only once shortly after emergence. These laboratory results provide at least partial explanations for the sex ratios observed in the field.