Some isogenic strains produced from a natural population at El Prat (Barcelona) were tested at different temperatures for total viability. Their hybrids were also tested at the same temperatures for the same trait. Hybrids proved to be more homcostatic than their isogenic parental strains. More efficient homeostasis in hybrids is suggested to be the cause of increased hybrid superiority with decreasing temperature. Genotype-temperature interaction was always significant for isogenic strains, but never for hybrids. These results are discussed in terms of coadaptation and homeostasis. Some maternal effects have also been detected. © 1970 Martinus Nijhoff.