Summary.1. The bridled form of the Common Guillemot(Uria aalge)is a single factor mutant, which is present in varying proportions over the North Atlantic colonies of that species.2.‐ Counts were taken, with the help of the British Trust for Ornithology, at the breeding cliffs during the summers of 1938 and 1939, and the proportions of bridled to normal birds ascertained.3. The full results show that the percentage of bridled birds increases from south to north and, in a less degree, from east to west. This cline in dimorph‐ratio is not even: in the British Isles the population shows sudden increases in the bridled percentage at two zones, the Mull of Kintyre and the channel between Pair Isle and Shetland. In Iceland the cline is reversed in sign and the percentage of bridled birds decreases again from the south to the north.4. This stage of dimorphism may be due to the unimpeded spread of an advantageous mutation, or it may be reaching a balanced condition owing to counter selection at various levels by environmental factors. This balance might or might not have been reached; a repetition of the inquiry after ten years should settle this question. Coincidence of the cline with an increase in humidity northwards and westwards is noted.5. Statistical analysis of ledge counts shows that there is no heterogeneity within the separate colonies, with the exception of Mingulay, where a special factor seems to be in operation. This suggests the absence of any marked degree of assortment. Fisher's hierarchicalx2test demonstrates the step‐like nature of the cline over the region from the Faeroes to the Hebrides. In particular, there is local heterogeneity but no general trend in dimorph‐ratio over the area stretching from Fair Isle to M