There are few basic studies on biology of coccinelids as predators of aphids and their potential as biological control agents. The purpose of this study was to establish the biology and morphology of Diomus sp (Coleoptera: Coccinellidae), a predator of the cowpea aphid, Aphis craccivora Koch (Hemiptera: Aphididae) under laboratory conditions. For this purpose, daily observations were made over the biological cycle from egg laying until the death of the adult in observation chambers. Under controlled environmental conditions the average time span of the egg stage and first, second, third and fourth stage of the larvae was 5.55, 2.06, 1. 29, 1.41 and 4.02 days, respectively. The average time span of the pupal stage was 5.70 days. From the egg stage up to the emergence of the adult elapsed 20 days. Of 42 sexed adults, 18 were female and 24 were male. The average longevity of the adult was 105.07 days. During reproduction the average time of preoviposition of Diomus sp virgin female was 4.8 days. The female Diomus sp layed an average of 352 eggs with a range between 155 to 784 eggs.