Considerable genetic variation in virulence, fecundity, and adult weight was found between and within clones of greenbug,Schizaphis graminum(Rondani), populations near Davis, Calif. Eight clones were established from individual apterous aphids collected from four fields within 50 km of each other. Aphids from these clones differed in their ability to damage wheat variety ‘TAM 107’ and in their weight and fecundity on several sorghum varieties. Based on these differences, the clones could be separated into four distinct groups. Lineages of four clones, maintained for 14–16 successive generations by selecting either large or small aphids, differed in their fecundities. These differences could not be explained by maternal effects. If biotypes of the greenbug are determined by the damage they cause to particular plant varieties, as recently proposed, then clones that differ in fitness traits such as fecundity and weight cannot be considered different biotypes. However, to develop strategies for reducing the rate of aphid adaptation to resistant varieties, we must understand such variation in fitness on a local scale, information that is not obtained in general biotype surveys.