C. lemniscatus is the only species of the genus Cnemidophorus found in the Amazonas valley. Among the populations from 15 localities which were studied, 5 included individuals of both sexes, but the remaining 10 were composed only by parthenogenetic females. In every case the all-female populations presented one or more heteromorphic pairs of chromosomes; in no case did this happen in the sexual populations. Five different karyotypes were described but no two of them were present in the same locality. No triploid individuals were found. A hypothesis for explaining this situation is discussed. It is probable that the chromosomes of the ancestral sexual population presented structural balanced polymorphisms preserved by heterosis or by better adaptation of each homozygote to a different niche. The latter case explains better the finding of different karyotypes without heteromorphism in different bisexual populations. When occasional parthenogenesis happened to appear in an heterotic structural heterozygote a successful strain could be established which would displace its bisexual competitors.