Twenty-four samples of anadromous brown trout from four river systems/geographical regions were studied by PCR-RFLP analysis of the ND-1 and ND-5/6 regions of the mitochondrial genome. A total of 14 haplotypes was observed, and these could be divided into three phylogenetic groups. Populations within river systems/regions tended to be more closely related to each other than to populations from other river systems/regions. Also, a significant correlation was observed between geographical and genetic distances between populations. These results contrast with results from other studies of mainly resident and landlocked populations, where no correspondence was observed between genetic relationships and geographical location of populations. Gene flow connecting anadromous populations is probably the reason for the observed isolation-by-distance patterns, whereas in isolated resident and landlocked trout populations drift leads to random genetic divergence of populations. Tests for nonrandom geographical distribution of phylogenetic groups of haplotypes showed that drift and gene flow are probably the predominant factors affecting the distribution of haplotypes. There were, however, also some indications of clines in frequencies of phylogenetic groups of haplotypes.