Comparative studies of melanism in the two cryptic moth species, Diurnea fagella (Denis & Schiffermüller) and Allophyes oxyacanthae (L.), have been carried out in southern England and south Wales. Estimates of the relative crypsis of the melanic and typical forms of both these species have been made at a number of sites and these were compared with the melanic frequencies in samples from these sites. These comparisons showed that selective prédation could be a major factor in the variation of melanic frequencies of both of these species. A consideration of the spread of melanism in these species suggests that non-visual selection may favour the melanics of D. fagella in urban areas and that non-visual selection, not closely associated with urban conditions, may be responsible for the restriction of melanics in A. oxyacanthae to Britain. The results for these two species are discussed in relation to investigations of melanism in other moth species.