Calathus melanocephalus is always more or less melanistic in Iceland. Tests were carried out to determine whether the degree of melanism was causally correlated with weather conditions during the immature stages. Samples from the same area collected in different months and years were compared. The correlation betweeen melanism and weather was slight, or even absent. A comparison of samples from different parts of Iceland showed a clear predominance of pale individuals in the east. This can only be explained as being due to genetic differences between populations.