SUMMARYSamples of the seaweed fly, Coelopa frigida, from six populations widely spaced around the English coast were collected during 1974-76 and again in 1979-80. Gene frequencies at the Alcohol dehydrogenase (Ad/i) locus were found not to vary significantly between sites, nor over the five year period of their study. Genotype distributions are compared with Hardy-Weinberg expectations and show consistent, mostly significant, differences.These results are interpreted in the light of an association between the Adh locus and a chromosomal inversion polymorphism. It is argued that the remarkable geographical uniformity must indicate the presence of strong selection, probably taking the form of heterokaryotypic advantage.