Danaus chrysippus was reared in the laboratory from stock obtained from Kampala (Uganda), Nairobi (Kenya), Dar-es-Salaam (Tanzania) and Freetown (Sierra Leone) and wild-caught samples from Nigeria, southwest Africa and Tanzania were also analysed. D. plexippus was reared on the same plants for comparison. It was found that the adult D. chrysippus is a poor and inconsistent storer of cardiac glycosides compared with D. plexippus, and contained, chiefly, highly polar cardenolides. Populations in East Africa are, on the whole, more efficient storers than those from West Africa, a factor which may contribute to the dearth of mimics in West Africa. Furthermore there appears to be a genetic element in the storage capacity of these butterflies, not merely a 'mirror' effect, depending on the cardenolide content of their food plants. Differences in storage capacity were shown between morphs alcippus from both Sierra Leone and Dar-es-Salaam and aegyptius from Tanzania and Dar-es-Salaam, reared side by side on the same tested food plants. In both cases aegyptius was the better storer but in other broods from Kenya, reared on Asclepias rich in cardenolides, this morph was negative for these substances. During the investigation strains of Asclepias curassavica were found which contained calotropin, but lacked calactin. D. plexippus reared on these plants also lacked calactin, but sequestered and stored it when fed on Gomphocarpus fruticosus which contained both substances. The methods for analysing cardenolides of this type are described. Maps are presented showing the distribution of the three principal morphs of D. chrysippus and the form albinus in Africa.