The butterfly, Acraea encedon, is less polymorphic in West than in East Africa. Field sampling in seven populations indicates that mimetic forms, which are common in East Africa, are rare in West Africa. Colour forms lycia and commixta are allelic, and lycia is completely recessive. There is no evidence of heterosis. The gene frequency in one population in Sierra Leone fluctuated about a mean value, and after a dry season during which the butterflies were scarce, the initial gene frequencies were significantly different from those obtained earlier, but eventually stabilized at the original level. Differential survival of batches of eggs and larvae may account for the fluctuation in gene frequency. Although the frequency of the colour forms in one population was stabilized over about 13 successive and overlapping breeding generations nothing is known of the ecological factors that impart this stability.