Abstract Karyotypic characteristics of the chironomids Demeijerea rufipes (Linnaeus, 1761) and Xenochironomus sp., obligatory miners of freshwater sponges, are considered. The karyotype of Xenochironomus sp. is described for the first time. It is assumed that the ordered spatial organization of chromosomes of these species, expressed in terms of the presence of chromocentres of different degrees of stability and an almost complete absence of polymorphism for chromosome rearrangements, represents an adaptation to inhabiting host tissues, i.e. to habitat relative constancy.