Interspecific interactions have been shown to play critical roles in determining the realized niches of species in marine rocky intertidal (ie Connell 1961, Paine 1966,, Dayton 1971) and terrestrial (i.e~ Harper 1969, Pianka 1973, Cody 1974) communities. In contrast, such interactions have not been clearly demonstrated as determinants of the distribution and abundance of marine intertidal soft-substrate organisms (however, see Levinton 1977). There are great physical differences between the rocky and mud-sandflat intertidal habitats, Some of these are wave exposure, sediment size (boulders to silts), penetrability and organic content of the substrate, frequency of log damage, and exposure to sunlight. Considering these physical differences and the fact that soft-substrate environments often support diverse communities, it is interesting to assess the role of biological interactions in' structuring these intertidal communities. Previous work on this system primarily consists of descriptions of animal-sediment · relationships and of food partitioning (i.e~ Sanders et al.. 1962, Mangum 1964, Reid and Reid 1968, Johnson 1971, Rhoads and Young 1971, Young and Rhoads 1971, Fenchel et al, 1975, Warren 1977). The role of interspecific interactions in establishing and maintaining infaunal distribution patterns on a sandflat in northern California is reported here.