SUMMARYKnowing the genetic regulation of fitness is crucial for using mutants in breeding programmes, particularly when the mutant is deleterious in some genetic backgrounds, as it happens with the sweet corn mutant sugary1 (su1) in maize (Zea mays L.). The fitness and genetic effects of maize mutant su1 were monitored through five successive selfing generations in two separated mean-generation designs. The first involved two inbreds with similar genetic backgrounds, while unrelated inbreds were used for the second design. Parents, F1s, F2s, and backcrosses were crossed to P39 as the donor of su1 and the 12 crosses were successively self-pollinated for 5 years. The su1 frequency decreased linearly across selfing generations in both designs. Additive effects were significant for su1 seed viability. However, dominance effects were of higher magnitude than additive effects, even though the dominance effects were not significant. Genetic effects depended on genotypes and environments. Therefore, the fitness of su1 is under genetic control, with significant additive effects due to minor contributions of multiple genes. The fitness of su1 is strongly affected by maize genotypic background and environment. It is hypothesized that genotypes could have evolutionary potential for modulating the fitness of single mutations.