Black-footed Penguins, Spheniscus demersus, have been living in an open air enclosure in Artiszoo since 1961. Their numbers varied from 7 to 103 in the period under study extending from 1961 to 1982. The information used in this survey is derived from records made by the zoo keepers and from a study of the behaviour of the penguins that was performed in 1979-1980. The pair bond between breeding birds appears to be very strong, the only bird that ever disassociated itself returned to her first partner after one year. However, the penguins seem to find a new partner in a very short time if they happen to forfeit their first partner. The couples have a strong tendency to breed each season in the same burrow. The occasional shifting to other burrows seems not to be related to the fate of the first clutch. The partners stayed together in nearly all cases in which breeding birds changed burrows. A burrow seems to get new owners only when the previous couple vacates it. This has had the consequence that, in some years, young couples could not install themselves because there was a lack of nesting places. The clutch size is two and the number of clutches per season is one or two, three is less common. The birds are probably encouraged to lay a second or third clutch when the previous one fails visibly in an early stage. The frequency of laying second and third clutches might decrease if the penguins in Artiszoo were allowed to revert to their natural cycle of guarding their young for 80 days instead of the enforced period of only 42 days. The breeding season runs from August to May and has two peak periods of egg-laying, one in August/September and one, less extreme, in December. The timing of breeding varied from year to year, in some years the first egg-production peak appeared in July/August and in others it appeared only in October. The penguins in Artiszoo start breeding for the first time when they are two years or older, just like the penguins in South Africa. Since 1965 the population growth has been caused entirely by the reproductive qualities of 19 birds and their descendants. The hatching success of eggs decreased spectacularly in the years after 1971 when the number of available adults exceeded the figure 25, and since that time relatively more eggs disappeared or were found to be broken. This study shows that the decline of the hatching success is caused both by a lack of nesting places and the increase of the number of penguins living in the enclosure.