Morphological variation in six North American populations of three Aphidius species parasitic on the pea aphid was examined. With the aid of multiple discriminant analysis and cluster analysis of the Mahalanobis generalized distances between populations, the six populations were separated into two distinct groups: One group consisted of A. ervi Haliday, introduced from Europe, and the closely related North American A. pisivorus Smith; and the second group consisted of A. smithi Sharma and Subba Rao, originally introduced from India. The findings are discussed with regard to the colonization history of Aphidius species in North America and biological control.