The butterfly, Acraea quirina, occurs as an adult in all months of the year in forests around Freetown in Sierra Leone. For ninc months (July— March) it is not common but in April—June, at the onset of the wet season, becomes exceedingly abundant. During this period there are two generations, the first containing mainly females, while in the second there is what is probably a normal sex ratio. In butterflies the sex ratio is usually 1 : 1 but a number of instances of an excess of females are now known. The possible mechanisms and significance of distorted sex ratios in butterflies are discussed, not only for A. quirina, but for other species, including Hypolimnas bolina.