The range of concentration of calcium carbonate in the shells of various freshwater gastropods was determined using ion chromatography. Individuals of Helisoma trivolvis (wild) were collected from three ponds in Pennsylvania and New Jersey; individuals of Physa sp. were collected from one pond in New Jersey; individuals of H. trivolvis (Colorado strain) and Biomphalaria glabrata (NMRI strain) were raised in the laboratory; and individuals of Pomacea bridgesii were purchased commercially. The concentrations of calcium carbonate (mean % by dry weight) of the shells were as follows: H. trivolvis (wild), 97.0; Physa sp., 97.8; H. trivolvis (CO), 97.6; B. glabrata, 98.8; P. bridgesii, 98.2. Our data support and validate the previous claim that snail shells are comprised of 95–99.9% calcium carbonate.