Partula gibba, commonly known as the Fat Guam Partula, is endemic to Guam and Northern Mariana Islands, and it is listed as endangered in the IUCN Red List of Threatened Species. The effects of calcium concentration on its growth and reproductive success were studied. The snails were divided into six groups, fed ad libitum on a diet based on the one formulated by Frame and Clarke at the Nottingham University approximately 25 years ago or variants in which cuttlebone was substituted by various concentrations of calcium carbonate. Shell height, thickness and width, aperture/lip height and width where recorded every month for a whole year, and all were found to be significantly positively correlated with the percentage of added calcium carbonate in the diet (range 0–40% dry wt:wt). Morphometric growth parameters of snails fed 20% added calcium carbonate did not differ significantly from the diet with added cuttlebone (20%), with the exception of their reproductive success, which was lower with the cuttlebone treatment. Reproduction success (expressed by the number of newborns per adult per month) was also significantly and positively correlated over the range of added calcium carbonate to the diet.