The Brown Hairstreak (Theda hetulae) is a scarce and endangered butterfly in Flanders. Information about the ecology and distribution of the species in Flanders is given. Monitoring the Brown Hairstreak is not easy as the elusive butterflies are rarely seen. Counting the eggs on the tivigs of blackthorn in winter is a better method to determine the distribution of the species. This was done during three consecutive winters in the region between Brussels and Tienen. Results showed that colonies of the Brown Hairstreak are larger and more widespread than was known before. No conclusions, Iwwever, can he made about the increase or decrease of the species, as earlier data were based on observations of adult butterflies alone.