Each month (except March 2012), we collected detailed data on the 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 50 m 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 (11,643 dissections total, 5,886 of which were infected with at least one trematode species), describing trematode infections to one of 20 species. To collect snails for dissections, 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). For lattitude and longitude coordinates of sites please refer to: Buck, J.C., Wood, A.C., Cook, I.M., and Lafferty, K.D., (in press), Parasite Recruitment and Host Risk in a Snail-Trematode System at Carpinteria Salt Marsh: U.S. Geological Survey data release, http://dx.doi.org/10.5066/F7GX48P2. References: Kuris, A. M., R. F. Hechinger, J. C. Shaw, K. L. Whitney, L. Aguirre-Macedo, C. A. Boch, A. P. Dobson, E. J. Dunham, B. L. Fredensborg, T. C. Huspeni, J. Lorda, L. Mababa, F. Mancini, A. B. Mora, M. Pickering, N. L. Talhouk, M. E. Torchin, and K. D. Lafferty. 2008. Ecosystem energetic implications of parasite and free-living biomass in three estuaries. Nature 454:515-518.