-Chironomid larvae were collected from different localities of industrial polluted area of Finland. By the cytogenetical and external larval morphological analysis the material studied could be referred to either as Chironomus plumosus L. or as C. anthracinus Zett. The effect of an industrial and a municipal contamination on the appearance of structural and functional alterations in the polytene chromosomes of both studied species was shown. Chromosome and genome polymorphism of Chironomus plumosus L. in different localities of industrial polluted areas of Lohjanjaveri, south Finland is described. A high chromosome polymorphism involving para-and pericentric inversions was established. A polymorphic system exists in all studied localities and it is characterised by the transition of one homozygous inversion into another through heterozygote formation. The studying areas were characterised by an increasing frequency of inverted homozygous inversions in arms A,C,D. The genome polymorphism is expressed by an additional " B" chromosome which varies in frequency and morphology. This polymorphism may provide the population with better chance to surviving under extreme polluted conditions. Structural-functional alterations in C. anthracinus were observed. New para and pericentric inversions, were found in the stud ied polluted areas. Balbiani ring system (BR) of C.plumosus and nucleolus (N) of C. anthra cinus were very sensitive to the pollution. Together with a normal activity of BR system of C.plumosus and N of C.anthracinus we observed a clear reversed level: there were number of individuals and cells which BR and N were in heterozygous state or collapsed. Their poten tial for detecting genotoxic agents in ecosystems are discussed and some morphological de-, • ' formities of both studied species were described. However, no correlation between the rate ,> of chromosome rearrangements and rate of morphological deformities were established.