An individual ofStauroderus scalariswith a heterozygous reduction of the L3procentric heterochromatin has been found in a Spanish population. Chiasma frequency and distribution have been studied in the three long pairs of the complement and compared with those of a normal individual. Although there are no significant differences in the L3bivalent chiasma frequency, it has been observed that chiasmata move away from the centromere in the heterozygous individual giving a different pattern of chiasma distribution with respect to a normal individual. Different aspects related to chiasma frequency and distribution in other bivalents are considered.