Spatial variability in abundance and age distnbution of 2 infaunal bivalves, Cerastoderma edule (Linnaeus, 1785) and C. lamarcki (Reeve, 1845), were quantified using hierarchical sampling programs. The scales investigated ranged between 10' and 10' m. Analyses of variance revealed significant vanability in abundance among sites within bays (-100 m apart) for both species and also among bays (-1 km apart) for C. lamarcki. Mean age vaned at several spatiai scales. For both species, however, the scale contributing most to the variation was bays, which provided about 40 % of the total vanation for each species. Randomization procedures were used to test the generality of previous observations of non-random distributions of year classes (namely number of year classes and dorninance by Single year classes). These analyses showed that for both species a spatial unit contained significantly fewer year classes and the most abundant year class was significantly more abundant than expected by chance. These results were consistent for ali spatiai scales except among plots (-10' m apart) in C. lamarcki. The necessity to assess spatial patterns at a wide range of spatial scales is emphasized. Potentially important processes (focusing on larval supply and recruitment) which generate the observed patterns and general implications of spatial vanability are discussed.