The melanic forms of Biston betularia and Gonodontis bidentata are common in northwest England. Kettlewell suggested that such morphs are better camouflaged from birds that seek them as food while they rest by day on exposed surfaces blackened by air pollution. He demonstrated that in an urban area the carbonaria melanic of B. betularia survived longer than the relatively conspicuous non-melanic typical form, and that this situation was reversed in an unpolluted area. Capture-recapture methods can be used to estimate daily survival rates and size of natural populations. These techniques were applied to Biston betularia at two localities and Gonodontis bidentata at five localities in northwest England and north Wales. The methods of Fisher & Ford (1947), Jolly (1965), Manly (1973) and Seber (1973) were used to analyse field data. Daily survival rates of the morphs, their expectations of life and their relative fitnesses were estimated. The rate of loss (1-survival rate) is a complex parameter including death and permanent emigration. There was no evidence for differential rates of movement of the morphs of a species, so differences in loss are due to selective death of morphs, probably arising as a result of predation by birds. In the study area the data for Biston betularia, together with the results of Bishop (1972) for seven other localities, present an unequivocal picture. There is a significant regression of the estimated fitness of typical relative to carbonaria at a locality and the frequency of typical in samples from that locality. Previous work has shown a strong correlation between the frequency of typical and the number of lichen taxa present on oak trees. There is no similar regression of fitness on frequency for Gonodontis bidentata although at Rusholme, central Manchester the evidence suggests that the non-melanic morph is at a disadvantage to the nigra form. The selective forces maintaining the polymorphism for nigra in G. bidentata are poorly understood. The density of populations of Gonodontis bidentata ranged from about 4:00 000 moths/ km<latex>$^2$</latex> per flying season in south Liverpool to about 8000 moths/km<latex>$^2$</latex> per season in central Manchester. Corresponding figures for B. betularia are probably less than 1000 moths/km<latex>$^2$</latex> per season. That species appears to be highly mobile, a substantial fraction of adult males leaving the area where they developed. The evidence available for G. bidentata suggests that it is very much less mobile. The differences in density and movement affect the relative rates of gene flow in the two species.