We collected detailed spatial data on the density and size distribution of intertidal snails as part of a broader effort to understand food webs in California estuaries. The survey area was Carpinteria Salt Marsh, California USA, which comprises 9 Ha tidal channels, 2 Ha salt flats, 17 Ha upland habitat, 6 Ha tidal pans, 52 Ha vegetated marsh, 2 Ha tidal flats. Using nearly 4,000 transects in potential snail habitat, we mapped snails throughout the estuary. Specifically, we systematically placed transects at intervals stratified within targeted habitat types: channels, pans, or marsh (or planar habitat that was mixed marsh and pan). At a quarter of the quadrats, we also estimated snail size-frequency distributions. Because we recorded the location of each quadrat, these data can be used to map the distribution of snails throughout this estuary.