Two populations of seaweed flies (Coelopa frigida) from the north-east coast of England were sampled at approximately monthly intervals. Single samples were also collected from populations on the south coast. The allele and genotype frequencies at the alcohol dehydrogenase locus, known to be associated with the large a/13 chromosomal inversion, were determined for each of the 34 samples.Aspects of the habitats were quantified at the time of sampling, as were 12 variables extracted from meteorological data. The genetic, ecological and climatic data are used to test predictions deriving from laboratory experiments and from previous, smaller scale, sampling programmes. There was an excess of heterokaryotypes over Hardy-Weinberg expectations which was stable over time. While the frequencies were close to the predicted equilibrium values, the rate of return following departures from the equilibrium was more rapid than predicted from the operation of heterokaryotypic advantage alone. Estimates of the effective population size were very much lower than census estimates. Karyotype frequencies cycled seasonally and were associated with both temperature and the incidence of gales. The interactions of ecological and life-historical factors with the observed genetical variation are discussed.