We examine the role of soil moisture and lipid reserves in determining the previously unknown vertical movements of the entomopathogenic nematode (EPN), Heterorhabditis marelatus (Rhabditida: Heterorhabditidae). Soil-dwelling EPNs require aqueous surroundings to move and avoid desiccation. The non-feeding H. marelatus infective juveniles are energetically limited in their ability to disperse, migrate, and infect new hosts by the lipid reserves they are endowed with at emergence. Our dry-season soil surveys recovered H. marelatus populations at depths of 50-100cm in disparate habitats. This contradicts the general assumption that EPNs are restricted to the upper 25cm of the soil column. Experimental results show that H. marelatus will climb an 80cm soil column at moisture concentrations above 8% and that a moisture cue is as effective as a host CO2 signal in triggering EPNs to ascend the soil column. Preliminary results suggest H. marelatus efficiently employ lipid reserves in locomotion, however as their membranes decay with age they rapidly deplete lipid stores by dedicating increasing amounts to osmoregulation. Thus, EPN infectivity and spatial ecology are determined by temporal and abiotic factors as much as energetic constraints. These results, combined with field monitoring, suggest that H. marelatus undergoes a seasonal vertical migration in the soil column with deep populations acting as source-sinks.