Surveys of unionid populations have been incorporated into the Swedish national environmental monitoring program. Nevertheless there is still lack of knowledge of important aspects of the biology of many unionid species. There may also be differences between species. This work compares the spatial distribution of two unionid species, Anodonta anatina and Unio tumidus, at four sites in Sweden. Samples were taken at transects along the shoreline at different water depths on the surface bed and in the sediments. Individual mussel ages were determined. Our results indicated that younger mussel individuals do not show differences in horizontal spatial distribution compared to older mussel individuals. However, they showed a preference to be burrowed in the sediment compared to older individuals that were predominantly found at the sediment surface. We also found a large difference in burrowing frequency between species with a higher frequency of burrowed adult individuals of A. anatina than U. tumidus. This result may be due to the timing of our field survey that coincided with the period of glochidia release for U. tumidus. We can conclude that a monitoring program for freshwater mussel populations needs to be carefully planned in time.