In August 1990 two butterfly species which have become extinct in The Netherlands, Maculinea teleius en M. nausithous, were reintroduced in a nature reserve in the province of Brabant. The author, who is engaged in this project, describes how the butterflies were collected from a stock population in southern Poland and later released in The Netherlands. Field research was done in 1990 and 1991 to see if the two species are able to establish themselves. Results showed that the population of M. teleius had increased, whereas the number of M. nausithous had decreased. The latter was thought to be due to the limited population of the host ant. Myrmica rubra, present in the nature reserve.