The importance of inbreeding depression in theoretical considerations of mating-system evolution(1-5) and its potential impact on the persistence of small populations(6) has renewed interest in the genetic basis of this phenomenon. Inbreeding increases homozygosity. This can produce inbreeding depression for two different reasons : first, deleterious recessive or partially recessive alleles that are masked at heterozygous loci by dominant alleles become fully expressed in homozygotes; and second, alleles may interact in an overdominant manner, such that the fitness of either type of homozygote is lower than that of heterozygotes. These two mechanisms produce different long-term effects in populations experiencing increased levels of inbreeding. Inbreeding depression resulting from deleterious alleles can be removed by selection, but inbreeding : depression produced by overdominance cannot be removed without lowering the mean fitness of the population(1-5). Using a North Carolina 3 breeding progamme(7), the most powerful quantitative genetics technique available(8-10), we show here that deleterious recessive alleles are mainly responsible for inbreeding depression in two closely related annual plants, the primarily selfing Mimulus micranthus and the mixed-mating M.guttatus. Estimates indicate that deleterious alleles in dir micranthus are more nearly additive than they are in M. guttatus.