It is generally believed that recombination by sexual reproduction is unfavourable in constant environments but is of adaptive value under changing environmental conditions. To test this theory, experimental populations of yeast (Saccharomyces cerevisiae) were set up and maintained at different levels of environmental heterogeneity. Recombination was estimated by determining sporulation rates. Sporulation rates first increased in populations living in highly variable environments, but after some time began to decrease. The decrease started last and was slowest in populations which were maintained under the same conditions for a sufficiently long time, to allow some adaptation of the gene pool to the respective environment. Patterns of genotypic variability could not be interpreted in such simple terms, but there was a statistically significant correlation between sporulation rate and genotypic variability. This correlation is to be expected because recombination generates genotypic variability. Summing up, recombination by sexual reproduction is advantageous in changing environments if the population can track the changes in the environment by changing its genotypic structure.