Information on the nature of epistasis between alleles affecting fitness is hardly available, but relevant for, among other issues, our understanding of the evolution of sex and recombination. Evidence of synergistic epistasis between deleterious mutations is support for the Mutational Deterministic hypothesis of the evolution of sex, while finding antagonistic epistasis between beneficial alleles would support the Environmental Deterministic hypotheses. Both types of epistasis are expected to cause negatively skewed fitness distributions of full-sib offspring from a sexual cross. Here, we have studied the form of the distribution of a variety of quantitative characters related to fitness by searching the literature. The fitness traits encountered include the mycelial growth rate in fungi, and earliness, resistance against pathogens, seed number, and pollen fitness in plants. Fitness-related traits in plants show almost exclusively negative skewness, while the results for fungal species are more ambiguous. Possible sources of negative skewness other than epistasis, such as recessiveness of deleterious alleles or a negatively skewed error variance, were tested and found to be unimportant. We argue that these results suggest the existence of synergistic epistasis between deleterious alleles or antagonistic epistasis between beneficial alleles in plants, which is general support for the currently popular hypotheses of sex and recombination, but does not distinguish between them.