Detailed observations on the life-cycle of Halictus marginatus are summarized. They show that the full biological cycle is that of a social species, and that it takes 5–6 years. Experiments with marked bees lead the author to conclude that castes (queen, worker) exist, and that caste determination takes place in the imaginai stage; it is an example of late epigenesis, the only one so far known. Halictus marginatus is primitive in nest building and its lack of social behaviour within the nest, and in the absence of morphological polymorphism. It is advanced in the longevity of the queen and her specialization in egg laying, in the perennial character of the nests, with periodical production of males, and in a functional polymorphism, with a probable inhibition of worker ovary development by the queen.