We are currently constructing a simulation of primary productivity, decomposition and nitrogen cycling in a Chihuahuan desert ecosystem (Moorhead et al. 1987). This ecosystem simulation model will be used to address hypotheses concerning mechanisms of interactions and feedbacks which exist between the various organisms involved in carbon and nitrogen flow in deserts. The soil organism community is an important component of this model because of its recognized role in regulating numerous processes, e.g., nitrogen mineralization (Swift et al. 1979, Anderson et al. 1985). In this paper, we present a descriptive submodel of the population dynamics of a general bacterivorous nematode as influenced by soil temperature and moisture. Recent investigations suggest that nematodes may play an important role in regulating decomposition processes and nutrient cycling by grazing on microflora and microfauna (Yeates 1979, Ingham et al. 1985).