The frequencies of the three phenotypes carbonaria, insularia and typical of the moth Biston betularia are given for a number of new localities and new samples from old localities in northwest England and the Isle of Man. In one locality (Caldy) the frequency of carbonaria has been decreasing at the rate of 1% per three years since 1959, probably due to the introduction of smokeless zones. Two methods have been used to measure the relative degree of camouflage at Caldy, where the melanic is common, and in the Isle of Man where it is much rarer. In Caldy carbonaria was the better camouflaged, but there was no evidence for this on the Isle of Man. One possible selective agent not previously reported is the bat, P. pipistrellus. The role of moth movement over land and water is discussed in relation to the maintenance of the polymorphism and the steepness of the clines between North Wales and Liverpool.