The Author has studied fresh-water planarians collected in a stream near Olbia - (Sardinia) and in the little islands Molara and Tavolara. All specimens of the three localities belong to Dugesia benazzii (a species of “gonocephala group”) but they differ as to their chromosome complements and reproductive mechanisms. Olbia population is triploid (24 chromosomes) with partially asynaptic oocytes, which show 8 bivalents and 8 univalents. The planarians from Molara are diploid (2 n = 16 , n = 8) with normally amphigonic reproduction. The specimens from Tavolara, on the contrary, are asexuated and reproduce esclusively by fission.