The karyotype ofZ. nasicumis described. It consists of 2n=20 ♂ (XY)-20 ♀ (XX), with a pair of large metacentric autosomes, a metacentric X and an acrocentric Y, the remaining autosomes being acrocentric. Both the metacentric autosomal pair and the XY♂- XX ♀ sex mechanism are present in all the species cytologically examined ofAleuasandZygoclistronexcept an undescribed species ofAleuaswhere the metacentric autosome dissociated into two acrocentric chromosomes. So, an ancient and common origin for both the fusions involved is suggested.