A population of fresh water planarians ascribable toDugesia gonocepbala s.l., characterized by individuals with markedly aneuploid chromosome sets was found near Sassari at Scala di Giocca in the Rio Bunnari. The most frequent chromosome number in 100 counts of metaphase plates of regenerative blastemata from the tail end of 7 different specimens proved to be 32, with a rather low frequency of 17%; after 32, the most frequent numbers were found to be 30, 31 and 27. Great numerical variations in the chromosome numbers were found, from a minimum of 16 to a maximum of more than 50. In all the sets with 32 chromosomes studied, 32 was never eutetraploid but always an aneuploid number; furthermore, a varying number of large submetacentric chromosomes was found in all the aneuploid sets examined. The authors point out that gynogenetic planarians with chromosome sets of 2n = 3x = 24 are to be found in the Rio Mascari, near the confluence of the Rio Bunnari, and amphigonic planarians with chromosome sets of 2n = 14; n = 7, characterized by a pair of large submetacentric chromosomes, identical to those in the aneuploid sets, are to be found in the Rio Bunnari. For these reasons the authors believe that the aneuploid population in the Rio Bunnari is in fact a stabilized natural hybrid originated from cross-breeding between individuals with 2n = 3x = 24 chromosomes, functioning as males, and individuals with 2n = 14; n = 7 chromosomes, functioning as females. It is suggested that the hybrid population might have reached its present stabilized condition according to the following succession of events: poly- ploidization of the initial hybrid chromosome set which thus became tetraploid (60 chromosomes); segregational mitoses of the neoblasts with 60 chromosomes resulting in various reductions in the chromosome number of the sets; assertion of the aneuploid sets in which chromosome numbers of 32, 30, 31 and 27 are most frequently found; reproduction by gynogenesis.