AbstractWith the view to study heterozygosity and heterogeneity as to their effects and interactions on yield and yield stability in maize, the following four types of population structure were formed from eight inbred lines: Homogeneous populations of homozygous plants (the eight inbred lines themselves); heterogeneous populations of homozygous plants (four blends containing four lines each); homogeneous populations of heterozygous plants (16 single crosses); heterogeneous populations of heterozygous plants (four blends containing four single crosses each, and four double crosses). This material was grown in three environments (years).At both levels of heterozygosity, the blends did not outyield the means of their respective components grown in pure stands. yield stability of the various structural groups was ranked differently by ecovalence and the mean square for deviations from regression. By transforming the deviation mean squares into coefficients of variation it was possible to reveal improvements of stability not only by heterogeneity but also by heterozygosity, and additionally, an interaction of both factors in the sense of a diminishing return.