C banding in polytene chromosomes of Chironomus riparius from a Santena polluted station of the river Po, Italy was described. A large variation in the appearance of C bands was established. The process involved the transformation of euchromatin into heterochromatin. Activation of all chromosomes at telomeres was found. A structural modification of telomeric heterochromatin may exist. The centromeric heterochromatin was rarely observed. A high percentage of an amplification (88%) in arm F, B3h, was recorded. These changes in the appearance of heterochromatin in the population studied could be considered as a type of polymorphic system, existing in the polluted population. For the first time in C. riparius two types of position effect variegation are reported. In arm E the euchromatin near to the centromere became inactive due to a heterozygous inversion and in arms D and F the opposite phenomenon was observed in which a heterozygous inversion partly suppressed the inactivation of heterochromatin.