Cytogenetic analysis of different populations ofBelostoma plebejum(2n= 14 + XY, male) has revealed the presence of two naturally occurring chromosome mutants: one heterozygous for an autosomal fusion, and the other with a sex chromosome fragmentation. Meiosis in both individuals is very regular. The fusion heterozygote (2n= 13 + XY) presented an autosomal trivalent at meiosis I, which divided reductionally at first anaphase; cells at meiosis II had 6 or 7 autosomes plus the XY pseudo-bivalent. The other chromosome mutant presented three sex chromosomes, which probably resulted from the fragmentation of the original X chromosome (2n= 14 + X1X2Y). The three sex chromosomes divided equationally at anaphase I and associated in a chain at metaphase II. Many authors are of the opinion that karyotype evolution in Heteroptera has proceeded principally by fusions and fragmentations. Particularly inBelostoma, it has been proposed that evolution from the ancestral number 2n= 26 + XY has occurred through autosomal fusions and an X chromosome fragmentation. The facts that these rearrangements have been found in natural populations ofB. plebejumand that they show a regular meiotic behaviour, seem to support the suggested hypothesis.