<br> Data on chromosomal polymorphism in two natural populations from the Inya River in Western Siberia (Novosibirsk province) of<br> Chironomus<br> sp. J (Kiknadze, 1991) —one of the sibling species from the<br> Chironomus plumosus<br> group — are presented for the first time. The species belongs to the “thummi” cytocomplex with 2n = 8 and the arm’s combination AB CD EF G and is closely related to<br> Ch. nudiventris<br> Ryser, Scholl et Wülker, 1983, which has 2n = 6 with the arm’s combination AB CD GEF (a modified “thummi” cytocomplex). The main difference between these two species is the number of chromosomes, apart from that they only differ by the frequencies of banding sequences in arm A, and the presence or absence of some polymorphic inversions. The banding sequence pool of<br> Chironomus<br> sp. J consists of 15 banding sequences. Inversions were found in five chromosomal arms – A, B, D, E, F. The most polymorphic arms were B and D. Two studied populations differed by the level of chromosomal polymorphism with one population being completely monomorphic and the other showing high level of polymorphism with 62–65% of heterozygotes and 0.83–0.88 heterozygotic inversion per larva (depending on the year of collection). Comparison of banding sequences to other species from the group showed that<br> Chironomus<br> sp. J is indeed closest to<br> Ch. nudiventris<br> , with the cytogenetic distance of 0.058 or 0.471 depending on the method of calculation, which indicates that these two species are very closely related. The relationship between<br> Chironomus<br> sp. J and other species from the<br> Ch. plumosus<br> group was discussed.<br>
