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SUMMARY
Four Gastrimargus species, namely G. africanus, G. crassicollis, G. vitripennis and G. wahlbergi were cytogenetically studied. The male chromosome complement is 2n(♂) = 23 (XO) with all chromosomes acrocentric. One G. wahlbergi and three G. vitripennis males were carriers of a large, mitotically stable B-chromosome, three quarters the size of the X-chromosome. The chiasma frequencies of non-B males (group mean = 14.83) versus B-containing males (group mean = 13.64) of G. vitripennis were highly significant. The frequencies of the proximally located chiasmata are significantly different in the two groups. It seems as if the presence of these B-chromosomes causes a decrease in the total chiasma frequency. Three individuals have elastic constrictions in one of the larger chromosome pairs. Two in the homozygous and one in the heterozygous condition.