We present stable isotopic analyses of collagen from 80 servicemen excavated from the late 18th/early 19th century naval hospitals at Plymouth (50) and Haslar, Gosport (30) in southern England. Historical records suggest that, the diets of these two populations should be essentially identical. While δ15N of the rib collagen confirmed that naval servicemen were relatively well-catered for in terms of meat allowance (Plymouth average δ15N = 11.1‰, Gosport = 11.9‰), stable carbon isotope analysis produced average values for the two assemblages, which were significantly different (Plymouth average δ13C = −18.8‰, Gosport = −20.0‰). We postulate that these differences stem from divergent naval postings, with a greater proportion of Plymouth individuals serving in areas that entailed a greater input of C4 foodstuffs. By comparison with published data from approximately contemporary burials at Snake Hill, Ontario, Canada and Chesapeake Bay, Virginia, we suggest that this area is the east coast of North America. For 15 of the 30 individuals from Gosport, we have data on ribs, femur, and dentine from the same skeleton, which appear to show that they came from a variety of locations in their preadolescence, but converged in dietary terms onto a “naval average,” which is consistent with historical evidence for recruitment patterns into the Navy at the time. By comparison with published data from skeletons recovered from the wreck of the Mary Rose (sank 1545), we conclude that this naval diet was virtually unchanged from the 16th century to the end of the 18th century. Am J Phys Anthropol, 2012. © 2012 Wiley Periodicals, Inc.