Aphidophagous coccinellid beetle populations were sampled by sweep net from 1969 to 1985 in small grain fields in eastern South Dakota. The assemblage of coccinellids consisted of six species:Hippodamia eonvergensGuerin-Meneville,H. tredecimpunctata tibialis(Say),H. parenthesis(Say),Coleomegilla maculata(DeGeer),Coccinella transversoguttataFaldermann, andCycloneda munda(Say).H. convergenswas the most abundant coccinellid, followed byH. tredecimpunctata,H. parenthesis,Coleomegilla maeulata,Coccinella transversoguttata, andCycloneda mundain rank order of abundance. Seasonal patterns of population increase differed among species. At all sites, populations of each species fluctuated unpredictably among years and independently of populations of other species. Assemblage structure at a site changed unpredictably from year to year, but the extent of change was constrained. Averaged over years, there were distinct differences in the structure of coccinellid species assemblages at geographically separated sites. Although assemblage structure varied from year to year at a site, there was no evidence of systematic change in structure at any site over the years of the study. A technique was proposed for evaluating the effect of introduced predators on coccinellid species assemblages.