Crosses between three open-pollinated varieties of maize exhibit heterosis in yield in the F1 generation and the F2 performance is intermediate between parents and F1. These findings are in contrast to those in Drosophila where the F2 has been found equal or inferior to parent strains in characters relating to fitness and they have been interpreted as evidence for the importance of epistasis. While the evidence does not favor important epistatic effects in the gene action conditioning yield in maize, the inconclusiveness of such results for its detection are recognized. The degree of genetic diversity of the parent varieties of maize with particular reference to the possibility of non-segregating loci in these variety crosses in contrast to populations developed from more divergent parents may account for the difference in results with Drosophila and maize.