Surveys of onion, Allium cepa L., fields in Colorado during 2004–2005 found that a complex of thrips species, comprised of onion thrips, Thrips tabaci Lindeman, and three Frankliniella species, were present in almost every location. Onion thrips predominated, averaging between 82.6 and 84.9% of the thrips recovered during the two seasons. Second in abundance was western flower thrips, F. occidentalis Pergande, that averaged 12.6 and 12.2% of the total thrips found during the two years, respectively. Two additional plant-feeding species, F. tenuicornis (Uzel) and F. schultzei (Trybom), also were recovered from the onion fields, and the latter comprised as much as 42% of the thrips in one southeastern Colorado field survey. The relative proportion of the thrips species in fields varied between sites and during the season, differences that might need to be considered when managing thrips on the onion crop. Three species, F. tenuicornis, F. schultzei, and a predatory species, the vespiform thrips, Franklinothrips vespiformis (Crawford), recovered during this study constitute new state records.