In our Institute Cynomolgus monkeys (M. fascicularis) are mainly used for production and control of poliovaccine. In 1974 a small individual breeding colony was started. Last year the colony was enlarged with 10 harem groups. Based on colony reproduction data, group breeding seems more successful than individual breeding. Owing to developments in the production of poliovaccine, a state of total self supply is within reach. However attention must be paid to factors reducing breeding results, comprising aggressive interactions and disturbed mother-care. Finally replacement of breeding stock might require rearing conditions which are not compatible with those necessary for the establishment of a Foamy virus and Herpes-B virus breeding colony.