This paper reports on the chromosome variations which occurred during a prolonged laboratory culture in specimens of the freshwater planarian Dugesia benazzii collected in the river Cavo (Corsica). In natural conditions, as well as in the first period of culture (1968-1970), all individuals were sexual and eudiploid in both somatic and germ cells. In 1971-1972, however, they became agamic and fissiparous. Their somatic cells showed a hyperdiploid complement of 19-26 chromosomes, which appeared homologous to the ones of the standard set of the species (A-chromosomes). More recently, B-chromosomes in a number of 1-3 and morphologically similar to those already described by the A. in other natural populations from Corsica, have been observed in 85% of the hyperdiploid somatic cells. The origin of polysomy and the mechanism of its transmission as well as the factors determining the appearance of the B-chromosomes in this strain are still unknown. The increase in chromosome number preceded the appearance of B-chromosomes, but a causal relationship between these two events is not ascertained. It is also worthy of attention that these karyological variations occurred after a long period of laboratory culture and at the time that fissioning was acquired. Aneuploidy has been frequently observed in fissiparous species of the "D. gonocephala group"; however, the correlation between these two phenomena is not constant and probably depends on specific genetic factors.