Aspergillosis was the commonest cause of death in gentoo penguins (Pygoscelis papua) at Edinburgh zoo from 1964 to 1988. Chicks were the most susceptible group and 14 per cent of hatched birds died of the disease (47 per cent of post mortem diagnoses) although in two years no cases were recorded and in two other years there were outbreaks which killed more than 50 per cent of the birds hatched. There was no correlation between the size of the colony and the prevalence of the disease but the climate during the rearing and weaning periods may have influenced the prevalence. The majority of cases of aspergillosis occurred between July and September and affected two- to three-month-old chicks shortly after they had been moved from their parents to a creche area for hand-feeding. No links were found between the occurrence of the disease and the date of hatching or the nest site, or whether the chicks were single, first or second-hatched twins, or with their weights, but the progeny of one parental pair were found to be overrepresented and more males than females died of the disease. Adult cases occurred sporadically and males were significantly more susceptible than females (P less than 0.05). Neither vaccination with a killed suspension of Aspergillus fumigatus nor therapeutic treatment with ketoconazole were effective in reducing the incidence of aspergillosis, although the late diagnosis of the disease was probably the main cause for the failure of these treatments.