The hermaphroditic flatworm, Polycelis nigra, is characterized by two reproductive biotypes which differ with respect to ploidy; sexual individuals are diploid (n = 8, 2× = 16) and pseudogamous parthenogenetic individuals are polyploid (typically 3×). We have collected and karyotyped individuals from 15 sampling sites (13 in mid to northern Italy, one in Great Britain and one in The Netherlands). We found that biotypes can exist alone or in sympatry, and identified purely diploid, mixed diploid-polyploid, and purely polyploid populations. Karyotype data show that in addition to the normal autosome complement, B chromosomes of differing morphology as well as stable aneuploid chromosomes (extra-A) were found almost exclusively in polyploids (11 of 12 sites). We extensively sampled Lago di Toblino (northern Italy), a pure polyploid population characterized by a submetacentric to metacentric, mitotically stable B chromosome, as well as a stable extra-A chromosome. Here, individuals having 1-3 B chromosomes were more abundant (61%) than those having no B's, implying that B chromosome infection has little detrimental effect when occurring in low numbers. Furthermore, 66% of individuals from this population possessed extra-A chromosomes, although it is unclear whether these elements are aneuploid autosomes or B chromosomes of different morphology. The ubiquity of these chromosomes, within asexuals in particular, is suggestive of a correlation between the origination of the elements and the evolution of polyploidy, or may reflect increased tolerance of parthenogenetic genomes to aneuploidy.