The frequencies of two hereditary coat colour types were determined in a sample of weasels, Mustela nivalis (n=1280) from the Czech and Slovak Republics. White pelage was found in four (0.49%) individuals collected in the period between October and April (n=824). All the other individuals studied had type II vulgaris coloration, characterised by an irregular border between the upper brown part of the body and the white underbelly. Distal white coloration on the feet and upper lip appeared to be unreliable in distinguishing between the two basic types of weasel summer coat. The karyotypes of five weasels from different parts of the Czech Republic contained large heterochromatic arms in six pairs of autosomes. The absence of a large heterochromatic arm in autosomal pair No. 7 differentiates the chromosomal complement of the central European weasels from those occurring in the northern parts of their range. The same karyotype found in Czech weasels was also found in an individual from European Turkey. Comparison of the available data on coat colour and chromosomal variation confirmes the existence of three major phylogenetic lineages of weasels in Europe.