The cytogenetic characteristics of species of the Chironomus plumosus group in Finland were examined. The species included C. balatonicus Devai, Wülker, Scholl, C. entis Shobanov, C. plumosus L., C. muratensis Ryser, Scholl, Wülker, and two karyotypes of unknown species. All belonged to the thummi complex having 2n = 8 chromosomes, with arm combinations of AB, CD, EF, G. In C. balatonicus arms A to G the band sequences corresponded with those of other populations. A new band sequence was found in arm B. In C. entis an arm A had a band sequence similar to those in C. agilis. A large pericentric inversion was observed in chromosome AB. The arms B to G had band sequences typical for C. entis. The chromosome arms A and B in C. plumosus from Lake Marsjon had band sequences corresponding to those of C. agilis and C. entis, respectively. C. plumosus from Helsinki may be a more divergent population with a large amount of centromere heterochromatin. C. muratensis was not distinguishable by band sequences from those of other Palaearctic populations. Two new karyotypes similar to those of species of the plumosus group have been described.