Three aspects of the biology ofCerotoma trifurcata(Forster), the bean leaf beetle, were studied: 1) the seasonal occurrence of adults and eggs in soybean fields, 2) the occurrence of adults and eggs on plants other than soybean, and 3) the effects on beetle populations of varying the phenology of the soybean.There are 3 peaks of adult abundance in central Illinois soybean fields; the arrival of overwintering adult colonizers and the 2 summer generations. The colonizers arrive shortly after soybean emergence in late May or early June and die after depositing eggs in the soil near the plants. First generation adults, present from mid or late July until the 2nd or 3rd wk of August, also oviposit and then die. Second generation adults are abundant in September and leave the fields when the plants are senescent. They eventually move to overwintering sites and do not oviposit until the following spring. Peak egg abundance always occurred later than peak adult abundance.Before soybeans emerged we found active adults but no eggs in alfalfa fields. Alfalfa may be an important interim food for adults in spring. We found eggs in the soil near 3 native wild Leguminosae,Desmodium illinoenseGray,D. cuspidatum(Muhl.), andStrophostyles helvola(L.).Changes in the phenological relationship between the soybean and the bean leaf beetle affect the duration of the plants' exposure to beetle injury, and may also have a significant effect on beetle population size. In Illinois, soybeans usually mature early enough to escape most defoliation by 2nd generation adults. Late planted fields escape the colonizers. General late planting over a large area can result in a great decrease in bean leaf beetle populations since, in the absence of their most abundant food plant most adults apparently die without ovipositing or at least without ovipositing on a suitable larval host.