Tapinoma melanocephalum has a haploid number of 5, comprising four metacentrics and one acrocentric chromosome. T. sessile karyotypes (n = 8) comprise seven metacentric chromosomes and one that has acrocentric, subacrocentric and metacentric alternatives. In some haploid sessile preparations with acrocentric or subacrocentric chromosomes present, an acromatic gap, sometimes with an interstitial granule, is seen in the long arm of the non-metacentric.