Three species of Bruchidae, Callosobruchus maculatus (F.), C. rhodesianus (Pic) and Bruchidius atrolineatus (Pic) were found associated with cowpea ( Vigna unguiculata) in Northern Nigeria. The infestation in the field was at a low level with an average of 2 per cent of the seeds being affected. B. atrolineatus was the most common of the three species and C. rhodesianus was comparatively scarce. Only C. maculatus continued infestation in store for any length of time. It is concluded that this was because B. atrolineatus and C. rhodesianus have a mean developmental period and a period of emergence which are longer than those for C. maculatus. In addition B. atrolineatus and C. rhodesianus have a much reduced level of egg production soon after the storage period begins. An account is given of the wild hosts of B. atrolineatus and it is concluded that Vigna triloba and possibly other legumes are primary hosts of this species whereas cowpea is not. Nearly ten per cent of the seeds of V. triloba were attacked in the field. Observations on the biology of C. maculatus in the laboratory showed that the period of pre-adult development is negatively correlated with temperature and is also affected to a lesser degree by humidity, low humidity lengthening the pre-adult developmental period. The majority of eggs were laid during the early part of the female's life and there was a decrease in the viability of eggs laid later. C. maculatus has been bred on several hosts, but the only host comparable to cowpea is Phaseolus aureus, other hosts lead to heavy mortality and extension of developmental period. There are differences in the susceptibility to attack of different cowpea varieties, the larger seeded varieties of those tested being, in general, better as hosts. This was shown not to be due to larval competition within the seeds. There was a definite preference for oviposition on seeds with a smooth testa when there is a choice of smooth and rough, but there seemed to be no colour preference. One insect can cause a 3–5 per cent weight loss in a cowpea seed. The decrease in weight loss per individual insect is lower when there are several larvae per seed. Germination is adversely affected when there are three or more insects per seed.