Interpopulation single crosses (F1's) and the progenies from one generation of selfing (S1's) arising from 128 unselected lines (64 from the maize variety ‘Jarvis Golden Prolific’ and 64 from the variety ‘Indian Chief’) were evaluated to determine the importance of epistasis. Significant epistatic effects were detected in some specific sets of crosses. However, the amount of the total variability that could be attributed to epistasis was, on the average, less than 10%. These and earlier results suggest that epistasis may be important in unique genetic combinations but these combinations occur either too infrequently or with such limited effect that they are not detectable in random mating equilibrium populations.