For a newly arising mutation affecting a trait under selection, its degree of dominance relative to the preexisting allele(s) strongly influences its evolutionary impact. We have estimated dominance parameters for spontaneous mutations in a subset of lines derived from a highly inbred founder of Arabidopsis thaliana by at least 17 generations of mutation accumulation (MA). The labor-intensive nature of the crosses and the anticipated subtlety of effects limited the number of MA lines included in this study to 8. Each MA line was selfed and reciprocally crossed to plants representing the founder genotype, and progeny were assayed in the greenhouse. Significant mutational effects on reproductive fitness included a recessive fitness-enhancing effect in one line and fitness-reducing effects, one additive and the other slightly recessive. Mutations conferring earlier phenology or smaller leaves were significantly recessive. For effects increasing leaf number and reducing height at flowering, additive gene action accounted for the expression of the traits. The sole example of a significantly dominant mutational effect delayed phenology. Our findings of recessive action of a fitness-enhancing mutational effect and additive action of a deleterious effect counter a common expectation of (partial) dominance of alleles that increase fitness, but the frequency of occurrence of such mutations is unknown.