Under certain conditions, Callosobruchus maculatus (F.) produces males and females which are distinct from the normal ones. These are called here the “ active ” form, and in southern Nigeria they occur most commonly during the months March to June. The active form has a longer pre-adult stage than has the normal form and it flies readily, unlike the normal form. There is some evidence which indicates that a genetic factor is involved in the production of the active form but no clear indication as to what external factors are involved. The normal female has mature eggs in its calyces within a day of emergence, and will lay about 60 eggs. The active female was found to have immature ovaries on emergence and these did not develop to any great extent in the conditions employed. About three-quarters of the active females never produce any eggs, and the rest produce an average of seven. This difference in egg-production probably makes food reserves available which are used to prolong the life of the active female. The eggs laid by active females are rarely fertile, and when adults do emerge they are apparently normal males and females.