Northwest England is one of the oldest industrial regions in the world and has suffered intense air pollution since early in the nineteenth century. Soot has blackened trees and walls in the area and sulphur dioxide has killed most of the lichens that were originally present. Night-flying moths, in particular the Peppered Moth Biston betularia, rest on such surfaces by day and are sought by birds as food. In 1848 a black, industrial melanic form of B. betularia was discovered near Manchester and by 1900 had almost completely replaced the light-coloured typical form. Kettlewell showed that this was because the carbonaria melanic was better camouflaged than typical when it rested on trees and walls affected by air pollution. In the last 25 years clean air policies have markedly reduced smoke and sulphur dioxide pollution in the region. A polymorphism with visually distinct forms of B. betularia exists in northwest England and adjacent areas of Wales. These forms are determined by a series of multiple alleles, with the darkest phenotypes (carbonaria) dominant to the lighter (insularia) while typical is recessive. Carbonaria is by far the commonest melanic in the northwest and is most abundant in the populations of B. betularia in Greater Manchester and Merseyside and is relatively rare in northern Wales. Data for the frequency of carbonaria, insularia and typical from 158 sites, between Leeds in the east to beyond Bangor in the west, are tabulated. The date of collection and the map reference of the sampling site are given to facilitate prospective sampling as the environment continues to improve. Some evidence suggests that the increase in the frequency of typical moths at Caldy, Merseyside and in Central Manchester may be due to reductions in air pollution. Gonodontis bidentata is another geometrid moth with a melanic (nigra) dominant to the non-melanic. The intensity of pigmentation of the non-melanic varies and specimens from northwest England are darker than those from the south of the country. The species is secretive and in the study area does not rest on exposed surfaces. Nevertheless nigra shows much local variation in frequency; it is most common in urban districts and least in the intervening rural areas. (In this way it is unlike B. betularia where carbonaria remains at over 85% frequency throughout.) Superimposed on this pattern of local differentiation is a tendency of nigra to increase in frequency from west to east. In Liverpool the highest frequency attained is 45% whereas in Manchester it reaches 80%. Data from 112 sites are also tabulated. There is no evidence of a fall in the frequency of nigra associated with a decline in air pollution. Some results are given which suggest that adults emerge at different times in different parts of the survey area. The two spot ladybird Adalia bipunctata also shows increases in the proportion of melanics in populations that inhabit the inner urban areas. The advantage conferred by melanism is not completely understood since the species is warningly coloured and distasteful. Melanics may be at an advantage in the presence of certain pollutants or they may be favoured by the lower amounts of sunshine received by areas suffering air pollution. In the ladybird there has been a spectacular decline in the frequency of melanics in Merseyside between the late 1960s and 1976. (Data from 40 samples are given.) This rapid response would be expected if the environment is acting directly on the species rather than through changes in the background on which the species rests as is the case in B. betularia. The maintenance of the polymorphism for melanism in B. betularia is discussed. Three possibilities are examined in relation to the evidence available from the study area as well as from the rest of the United Kingdom: (1) heterozygous advantage is important irrespective of selective predation by birds, (2) there is a balance between the disadvantage of the melanic homozygote and the forces of selective predation, and (3) predation is the sole important selective agent while migration of animals from different environments occurs to produce polymorphism. At present the available evidence is insufficient to distinguish between these possibilities, the existence of heterozygous advantage, assumed to be of primary importance by other authors, has yet to be established.