Using the salivary gland chromosome technique, three neighboring populations of Chironomus were analyzed for the incidence of chromosome rearrangements. In all three populations, there were found six major intra-chromosomal rearrangements. Four of them are simple paracentric inversions, one paracentric tandem inversion and one 'bulb'-like puff structure corresponding to four heavily stained bands in its homologue. The different variations are combined at random regardless of whether they are located in separate chromosomes, or in separate arms of a chromosome, or on the same arm of a chromosome, indicating that all of the three populations are maintained in an equilibrium state. The relative frequencies of the variants are, however, different in different populations. This suggests that the three populations, though separated from each other by only a few miles, are at least partially isolated.