Predation by crabs and shorebirds on the salt-marsli snail Cerithidea californica Haldeman was investigated with laboratory feeding experiments, a field mark-recapture study, and by examining shorehird regurgitation pellets. In Bolinas Lagoon, the field site of the study, the primary predators of Cerithidea are the grapsid crab Pachygrapsus crassipes Rathbun and the willet Catoptrophorus semipalmatus Gmelin. In the laboratory, male Pachygrapsus of three different size classes were offered snails of six different shell lengths. The average and maximum sizes of consumed snails increased and the proportion of snails in the smallest size class that were eaten declined, with increasing crab size. In a separate comparison of male and female crabs, males ate larger snails on average than did females of equal size. Shell sculpture in the form of variccs appeared to reduce the rate of successful attack by crabs, but crabs circumvented this defense in several ways. Rates of predation by a natural population of Pachygrapsus on marked and released Cerithidea were measured over a 10-wk period in summer. The local crab population consumed marked snails of a wide range of sizes. Larger snails suffered lower rates of predation, however, there was no evidence of an absolutely invulnerable snail size. There was no difference in the rates of predation on snails that were parasitized by larval trematodes and those that were uninfected. Rates of predation on marked snails varied widely, and in a density-independent manner, among the 10 snail subpopulalions into which they had been released. The density of crabs living in the immediate vicinity of a snail subpopulation was the best predictor of local variation in predation rates. The sizes of Cerithidea preyed upon by willets were determined by measuring shells in willet regurgitation pellets. This shorebird consumes much smaller snails on average than does the local crab population. Considering the broad diet of willels and experimental results of others, it appears that predation by shorebirds can sometimes depress the density of small Cerithidea, but its impact is highly variable in space and lime and apparently density-independent. Living snails were found in several willel regurgitation pellets, suggesting that transport in the crop/gizzard of a bird is a potential mechanism of Cerithidea dispersal within and among coastal lagoons.