Abstract.-Several recent theoretical considerations of mating-system evolution predict within-population covariation between levels of inbreeding depression and genetically controlled mating-system characters. If inbreeding depression is caused by deleterious recessive alleles, families with characters that promote self-pollination should show lower levels of inbreeding depression than families with characters that promote outcrossing. The converse is expected if inbreeding depression is due to overdominant allelic interactions. Whether these associations between mating-system and viability loci evolve will have important consequences for mating-system evolution. The evolution of selfing within the genus Mimulus is associated with a reduction in stigma-anther separation (i.e., a loss of herkogamy) and high autogamous seed set. In this study we compared families from two M. guttatus populations that differed genetically in their degree of stigma-anther separation. In one of these populations we also compared families that differed genetically in the degree to which they autogamously set seed in a pollinator-free greenhouse. Dams often differed significantly in levels of inbreeding depression for aboveground biomass and flower production, but variation in inbreeding depression was never explained by herkogamy class or autogamy class. Several factors might account for why families with traits associated with selfing did not show lower inbreeding depression, and these are discussed. Our study also demonstrated significant variation among self progeny from a given female likely due to differences in pollination date and position of fruit maturation. The detection of significant dam x sire interactions suggests biparental inbreeding or differences in combining ability for specific pairs of parents.Key words.-Combining ability, identity disequilibrium, inbreeding depression, maternal effects, mating system, Mimulus. Received February 6, 1996. Within a plant population, genotypes capable of self-pollination enjoy several potential advantages over outcrossing genotypes. If there is no reduction in their ability to serve as pollen donors in cross-pollinations, selfing variants have a 50% transmission advantage over strictly outcrossing genotypes (Fisher 1941). However, if self progeny are less than half as fit as outcross progeny due to inbreeding depression, the transmission bias is nullified and the spread of alleles that increase the selfing rate is prevented (e.g., Maynard Smith 1971; Charlesworth and Charlesworth 1979Lloyd 1979;Lande and Schemske 1985). Most empirical studies of mating-system evolution have focused on average levels of inbreeding depression within populations, and many have found that inbreeding depression often exceeds the 50% threshold required to prevent the invasion of selfing variants into outcrossing populations (e.g., Dudash 1990; Fenster 1991a; Dole and Ritland 1993). Only recently have studies begun to focus on genetic variation for inbreeding depression within populations (e.g., Pray and...