Respiration and food assimilation of two estuarine bacterivorous nematodes, the rhabditid Pellioditis marina and the monhysterid Diplolaimelloides meyli, were measured at a range of temperatures, salinities, and food densities. The aim of this study was to identify the fundamental niche of both species in their natural habitat, and to investigate the relative importance of food and abiotic factors in determining presence and success of nematode species in the highly dynamic estuarine tidal environments of macrophyte detrital habitats. Of the three factors studied, salinity least impacted P. marina and D. meyli. Respiration and assimilation in both species showed only minor variation in the salinity range of 10 to 30‰. Respiration decreased at marine salinities in P. marina, and increased in both species at oligohaline salinities down to a stress-induced maximum around a salinity of 5‰, then steeply declined towards freshwater conditions. Temperature heavily affected both species, but Q10-values in D. meyli were considerably higher than in P. marina, suggesting the former species to be particularly well adapted to fine-tuning its energy expenditure as a function of temperature. The highest respiration and assimilation rates in both species were at 25°C. At still higher temperatures, metabolic rates were depressed, but while P. marina was entirely inactivated above 30°C, D. meyli continued to respire and assimilate food up to 35°C. The scope for production, calculated as the net difference between assimilation and respiration rates (both expressed in units of C), was 0 at 5°C and increased to a maximum at 25°C in both nematodes; it declined at higher temperatures, but remained positive up to 35°C in D. meyli. Significant food assimilation in both nematodes occurred only at bacterial densities above 108 cells ml-1. Assimilation rate reached a maximum at 5×108 cells ml-1 in D. meyli, and remained constant at higher densities. P. marina, by contrast, had a well defined peak assimilation at a food density of 2.5×109cells ml-1, with lower rates at both lower and higher food densities. This contrasts with observations on ingestion rate, and suggests a food density-dependent assimilation efficiency. The present results suggest that tolerances of exposure to salinity and temperature extremes on a daily rather than seasonal basis, may be of higher significance in niche differentiation between both species. Their fundamental niche further appears to be determined by the range of (near) optimal food conditions, which is narrow in P. marina but comparatively broader in D. meyli.