1 Ants of the genus Dinoponera belong to a convergent group, in which there is no morphologically specialized caste of reproducing females and reproduction is by fertilized workers known as gamergates. Dinoponera lucida Emery, which is native to Brazilian Atlantic rain forest, is included on the official list of Brazilian fauna species threatened with extinction, due to habitat fragmentation, loss of their natural habitat and to peculiarities in their biology. 2 Karyotype variation was studied among D. lucida populations in the states of Bahia and Espirito Santo, Brazil. The cytogenetic study was carried out on brain ganglia and/or on male and/or female gonads. Banding techniques, such as sequential staining with DA/CMA₃/DAPI fluorochromes and FISH, were also applied. 3 The diploid chromosome numbers ranged from 106 to 120. Variations in the karyotype were detected in the populations from Bahia, while in Espirito Santo the karyotypes were the same everywhere. Most of the chromosomes were small in size and acrocentric, except for a differentiated pair (AM^t). This pair exhibited polymorphism in the different populations. 4 The karyotype variation detected in D. lucida suggested that the original population has previously been divided into two allopatric populations distributed in two different refugial areas of Atlantic rain forest in the early Quaternary. They probably developed in parallel and differentiated in karyotype composition. When the Atlantic rain forest regained its continuous distribution along with the Brazilian coast, the two groups came to occupy the continuous area of occurrence of the species that is seen today.