Abstract Kariological investigation of the earthworm Octodrilus transpadanus (Rosa, 1884) in the Ukraine showed that this species is represented by at least three races of different ploidy with the basic chromosome number n=15. Strict geographic heterogeneity of electrophoretic spectra of different populations was registered: south-eastern populations (7n=105) are characterized by a clonal structure, whereas western (from 3n = 45 to 4n = 60) and south-western (2n=30) ones have a high level of polymorphism. The Crimea sample (4n=60) is well isolated, has no clonal structure and is characterized by a series of specific alleles , which are absent in forms from continental Ukraine. All the forms turned out to be well isolated morphologically, except for the clonal forms, which are very much alike.