Heterosis often occurs in offspring derived from a cross between inbred or divergent parents and can be observed as the superior performance of these hybrids for a wide variety of characters. Heterosis was compared in maize lines at two ploidy levels, diploid and tetraploid, to gain a better understanding of the interaction of heterosis and ploidy level. Employing genetically identical diploid and tetraploid maize derived from four different inbred lines, we investigated heterosis for 11 morphological traits, including several plant height measures, as well as flowering time for both silks and anthers. We find that the heterotic response of a certain hybrid differs between diploid and tetraploid lines, and that the response at one ploidy cannot serve as a predictor for the other. Also, progressive heterosis was found for several of the characters in the tetraploid double-cross hybrid, which can have four different alleles at one locus, compared to the double-cross diploid hybrids, which can only possess two alleles per locus. Overall, the results indicate that the heterotic response of tetraploid maize lines differs significantly from that of the diploid. © 2007 Springer-Verlag.