An important step towards evaluating ecological sustainability of resource use is to understand the characteristics of that resource. Indonesian populations of the Asian water monitor (Varanus salvator) have been heavily exploited over several decades for the reptile skin industry. We visited two cities in northern Sumatra (Rantauprapat and Cikampak) to gather information on the sizes, sexes, reproductive status and food habits of harvested specimens. Data on 399 lizards showed that monitors in northern Sumatra are similar in most respects to those studied previously in southern Sumatra (Palembang), despite the considerable distance between the two areas and the significant difference in climates. Male water monitors mature at a smaller size than females, but grow to larger body sizes. Harvesting is concentrated on adult males, and adult plus juvenile females. Reproduction occurs year-round, but at a lower intensity in drier months (March and June). Females produce multiple clutches of 6-17 eggs each year. These lizards eat a wide variety of prey, including commensal vertebrates (e.g. rats, chickens) as well as invertebrates (e.g. insects, crabs). The numbers of stomach parasites (spirurid nematodes, Tanqua tiara) were higher in juvenile lizards than in adults, and varied between our two sampling sites. Our results suggest that water monitors exhibit relatively little morphological or ecological divergence over broad areas within Sumatra, hence simplifying the task of developing appropriate management systems for the commercial harvest.