The existence of temperature-dependent heterosls in maize and its contribution to hybrid vigor was investigated in controlled environments. Additional heterosis was found when plants were grown at temperatures above the growth optimum (33/27[degree]C), or following high-temperature shocks. This temperature-dependent heterosls was manifest because the hybrids grew relatively better than the inbreds at high temperatures, and recovered more rapidly after heat shock treatments. Temperature-dependent heterosis was also expressed at temperatures below 21/16[degree]C, which was due largely to the greater sensitivity of the inbreds to low temperatures. The results indicate that the level of heterosis demonstrated by hybrid maize is strongly temperature-dependent, and that the higher yields of the hybrids were due to the greater phenotypic stability over the entire temperature range. The genetic basis for high-temperature-dependent heterosis is thought to stem from the random fixation of genes sensitive to high temperatures in the inbreds, giving rise to thermolabile enzymes which lead to mutant expression only at elevated temperatures. This differential heterosis phenomenon may well provide a physiological interpretation for at least part of the heterosis observed in field-grown crops of maize which are subjected to high-temperature stresses.