Investigations have been carried out, both in the field and in the laboratory, into certain aspects of the biology of three species of millipede (Spirostreptus assiniensis, Oxydesmus sp., and Habrodesmus falx) which commonly occur during the rainy season in the Ibadan area of Western Nigeria. The feeding, mating, egg-laying, and moulting habits of the three species are described. A detailed description is given of the nest-building habit of the larvae of Oxydesmus sp. The elaborate nests which are built primarily for moulting also protect the larvae during the dry season when moisture conditions become unfavourable. Swarming behaviour, uncommon in millipedes, is shown by the larvae of H. falx. This behaviour which appears to be of survival value to the species is described.