Ten specimens of Tivela stultonim from Pismo and Torrance Beaches were analyzed for magnesium and strontium by atomic absorption techniques. Magnesium concentrations in the umbonal areas were consistently greater than those along the margins. Strontium concentrations remained relatively uniform throughout the individual shell. Maximum concentrations were 353 ± 21 ppm for magnesium and 2690 ± 300 ppm for strontium. Minimum concentrations were 114 ±21 ppm for magnesium and 1770 ± 300 ppm for strontium. Maximum additional magnesium possibly diffused into the shell material of two shells immersed in running sea water for three months was 134 ± 10 ppm. This apparent re-equilibration with sea water suggests that diagenesis can proceed rapidly after death of the animal.