The dispersion pattern of mussels belonging to the genus Anodonta has been studied in a shallow area of Lake Zurich. Starting in July 1970, mussels were counted six times at monthly intervals on a grid of 800 square units of 0.5 m2 each. The mean (mussels per m2) varied between months from 1.3 (in July and December) to 2.2 (in November). Different parts of the grid contained unequal numbers of mussels, which is explained by nonhomogeneousness of the substratum.The observed frequency distribution of number of mussels per unit showed a close fit to an negative binomial distribution. For further interpretation the index of dispersion as proposed by Morisita (1959) was computed, first using single units and thereafter combinations of 2, 4, ... units. From the slopes of the lines obtained (Fig. 3) it is concluded that the mussels form small aggregations, within which their distribution is random. Aggregation is most pronounced in July, during a season when male mussels are known to release their sperm.