Each month (except March 2012), we collected detailed data on the density, size distribution, and infection status of intertidal snails from ten fixed sites as part of a broader effort to understand food webs in California Estuaries. The study site was Carpinteria Salt Marsh Reserve, California USA, (University of California Natural Reserve System), 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. Each site was a fixed location in channel or flat habitat, with a diameter approximately 50m in size, centered at the site location marker used in Kuris et al. (2008). At each site/month, we collected, measured, sexed, and dissected ~58 snails. We collected and measured an additional ~42 snails for size-frequency distributions (7,793 snails total). Transects were placed randomly at flats sites. For channel sites, transects were placed randomly on the channel bank with the highest snail density. Each transect started 20 cm within the vegetated margin, and extended downslope to the deepest part of the channel or interior of the pan or to a maximum length of 10 m. For every 20 cm decrease in elevation, we established a new transect segment, or quadrat. 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.