Microsatellites are repetitive DNA sequences with high rate of slippage mutations, which cause changes in length. As expected by the neutral theory of molecular evolution the level of polymorphism of these sequences is high when located in non-coding regions that may experience little selection. We tested the hypothesis that triplet repeat microsatellites located in coding regions of the social amoeba Dictyostelium discoideum should have a low level of polymorphism, given the presumably stronger effect on fitness of changes in coding DNA. We analyzed the length of 8 microsatellites located in coding regions of Dictyostellium discoideum in 114 clones of the North America population, divided in 5 subpopulations. Our results showed that each of the eight loci was very highly variable in the population. The lowest range of length variation was 17 repeats (51 bp) for a microsatellite in the ATG1 gene and the maximum range was 78 repeats (234 bp) for a microsatellite in the dimA gene. We tested the possibility that the level of polymorphism was due to population structure. Although present, the population structure was low, and consequently not responsible for the high polymorphism.