Bean leaf beetle,Cerotoma trifurcata(Forster), adults were sampled from 1986 through 1988 in soybean and alfalfa in central Iowa to ascertain the seasonal phenology of this pest. Data from all 3 yr indicate that two generations of this insect occur in Iowa. Overwintering adults inhabit alfalfa before soybean emergence, colonize soybeans immediately after seedlings emerge, and die by late June. F1adults are abundant from late June to middle or late August, and F2adults are present from early August to soybean maturation. Females are more abundant than males among overwintering and F1adults, but neither sex of F2adults is more numerous. The yellow/red adult ratio is greater during the fall than in the spring, indicating greater winter survivorship of red adults. Adults were present in soybean from seedling emergence to crop maturation but were found in alfalfa only in late spring, before soybean emergence, and in August–September as soybean matured. Captures of adults during diurnal sweepnet collections made from 0830 to 2030 hours (CST) on 21 July 1988 were least at 0830 and 1000 hours, the period of heavy dew on the soybean canopy. From 1300 to 1600 hours, the number of females with developing ovaries declined, whereas the number of females with fully developed eggs increased.