Estimates of Felis catus coat colour gene frequencies were obtained from samples in 27 Scottish localities. These areas ranged from island samples in Shetland, Orkney and the Outer Hebrides to remote highland areas on the Scottish mainland. Animals from several towns and cities were also sampled. The results reveal considerable heterogeneity in the frequencies of the O, a, t b, l, S and W alleles. The frequencies of d were relatively homogeneous throughout Scotland. There were indications that island and mainland cat populations may have been derived from separate ancestral gene pools. Much of the observed gene frequency heterogeneity in rural areas could be attributed to differential migration or lack of penetrance. Geographical isolation and possible hybridization with F. sylvestris may explain the high frequency of wild-type alleles in mainland rural areas. A correlation between cat population "coefficient of darkness" and human population was observed. Such an observation supports the concept of "cumulative urban pressure" and indicates that a process analogous to industrial melanism is occurring in urban cat populations. The darkening of phenotype appears to be acting through the t, a and O loci and may be the result of pleiotropic effects of alleles at these loci.