Halictid bees are good systems for studying the evolution and maintenance of eusociality, because they form small societies where females have multiple behavioural options (stay or leave, reproduce or help). Here, we investigate colony organization, inter-nest movements and patterns of reproduction in Halictus scabiosae, a species where foundresses rear a first brood of females that often behave as helpers to rear a second brood. Using non-destructive sampling and microsatellite genotyping, we monitored the genotypic composition of a sample of colonies over the entire reproductive season, which allowed us to reconstruct sibships and infer parentage within and across colonies. We detected that foundresses and females from the first brood often moved to foreign colonies. Moreover, foundresses were frequently replaced. At least 5% of the females from the first brood reproduced. Eight of the 10 cases of reproduction by first brood females occurred in foreign colonies. Because of extensive bee movements, many colonies contained offspring from unrelated individuals. The average genetic relatedness among bees sampled from the same colony was moderate (0.33 ± 0.02). The relatedness of the second brood to their colony foundresses and first brood females was only 0.14 and 0.21, respectively. The labile colony membership decreases the intracolony relatedness and thus the inclusive fitness of helpers, but the behavioural flexibility of these bees may allow them to cope with variable environmental constraints.