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Chromosome-length measurements from five greenbug, Schizaphis graminum (Rondani), biotypes (B, C, E, F, and Maryland) were compared by discriminant analyses procedures. There were highly significant (P = 0.0001) differences in total lengths of chromosome sets among biotypes. In individual paired comparisons, where each biotype was compared with only one other biotype at a time, chromosome measurements correctly classified aphids to their respective biotype in 45-85% of the cases. Degree of correct classification was dependent on the two biotypes being compared and the discriminant function used in the analysis. Results indicate that there are two groups of biotypes that can be correctly distinguished from each other most of the time. Chromosome lengths of biotypes C and E were closely related. They could be distinguished from those of biotype Band F in 75-85% of the cases. The Maryland biotype appeared to be more closely related to C than to the other biotypes. There was considerable variation in the lengths of biotype C chromosomes, which suggests that biotype C may comprise several population variants, perhaps separate biotypes. The ranges of variability in length of chromosomes in biotype C overlapped the ranges of biotype E, which suggests that biotype E may be a selection from biotype C. Conclusions drawn from these data regarding relationships among biotypes are similar to those suggested by biological and plant-damage studies.