The genome of the social amoeba Dictyostelium discoideum is known to have a very high density of microsatellite repeats, including thousands of triplet microsatellite repeats in coding regions that apparently code for long runs of single amino acids. We used a mutation accumulation study to see if unusually high microsatellite mutation rates contribute to this pattern. There was a modest bias toward mutations that increase repeat number, but because upward mutations were smaller than downward ones, this did not lead to a net average increase in size. Longer microsatellites had higher mutation rates than shorter ones, but did not show greater directional bias. The most striking finding is that the overall mutation rate is the lowest reported for microsatellites: $1 3 10 À6 for 10 dinucleotide loci and 6 3 10 À6 for 52 trinucleotide loci (which were longer). High microsatellite mutation rates therefore do not explain the high incidence of microsatellites. The causal relation may in fact be reversed, with low mutation rates evolving to protect against deleterious fitness effects of mutation at the numerous microsatellites.M ICROSATELLITES, also known as simple sequence repeats, are long stretches of a short (1-6 bp), tandemly repeated DNA unit, such as the motif CAA repeated 20 times. Microsatellites are common throughout eukaryotic genomes and their lengths are often highly polymorphic, making them powerful markers for use in genetic mapping