Four homozygous mutant stocks and a wild-type stock (ORLANDO) of Anopheles quadrimaculatus were compared for susceptibility to Plasmodium yoelii. The mutant stocks, rose eye (ro) and red stripe (strd), had the same level of susceptibility as the control stock (ORLANDO), but Q2 (homozygous for brown body and nonstripe) and rose eye Q2 (roQ2), were significantly less susceptible. Body size of the adult mosquitoes had no effect on susceptibility.