A mathematical model of the population dynamics of the jackass penguin,Spheniscus demersus, is reported. The model is limited to a set of differential equations using fertility, mortality and recruitment rates to describe changes in four main population classes: eggs, unfledged chicks, immature birds and breeding adults. The value of the model lies in its availability for sensitivity studies of its various parameters, and in pointing out the directions which field research must take if many parameters are to be determined accurately.