We performed a mutation accumulation (MA) experiment using the social amoeba Dictyostelium discoideum to estimate the rate and distribution of effects of spontaneous mutations affecting eight putative fitness traits. We found that the per generation mutation rate for most fitness components is 0.0019 mutations per haploid genome per generation, or larger. This rate is an order of magnitude higher than estimates for fitness components in the unicellular eukaryote Saccharomyces cerevisiae, even though the base-pair substitution...