Rattus norvegicus (Norway rat) is one of the most abundant and geographically widespread mammals. Ecological studies focusing on urban Norway rat populations are restricted to temperate regions, and data describing populations from tropical cities are lacking. We compared the morphometrics and demographic characteristics of rats captured in urban low-income settlements in Salvador, Brazil, to a season, environmental area of capture, and sex-matched sample from Baltimore, Maryland. Norway rats of both sexes were significantly smaller in Salvador, although the threshold mass marking sexual maturity was equivalent for both cities. Pregnancy rates were comparable (∼50%); however, juveniles were commonly trapped in Salvador (20%) while they were rarely trapped in Baltimore during winter months (2%). These findings suggest that tropical and temperate rats differ with respect to body metrics, while size at sexual maturity is similar. Further studies conducted over different seasons are required before any firm conclusions are reached; however, this study provides preliminary support for Bergmann's rule that species at higher absolute latitude are larger than the same species sampled at lower latitudes.