A putative number of 27 loci were screened in six Swedish and five American samples of the freshwater crayfish Parifastacus leniusculus. The Swedish samples constituted three groups of populations originally derived from three different introductions from the USA and Canada. The American samples represented three subspecies. One of them, P. l. klamathensis, differed from all other samples by being monomorphic at the MDH-1* locus and displaying a high frequency of the IDHP-2*90 allele, which was absent in all other samples. These observations make any assumptions regarding influence From P. l. klamathensis to the Swedish populations groundless. The genetic identity estimated for polymorphic loci, between the populations introduced to Sweden and the other two subspecies, P. l. leniusculus and P. l. trowbridgii, was close to 1.0, thus implying that both these subspecies may have influenced the Swedish introductions. Populations derived from different source populations in Sweden did not demonstrate any signs of decreased genetic variability such as losses of alleles relative to their respective Swedish source populations. However, absence of rare alleles, viz. MDH-1*70 and PGM*105, in samples of two source populations, and observed skewed allele frequency in one at MDH-1* is probably due to founder effects caused by an initial stocking of too few individuals.
