Abiotic and biotic factors in soil modulate the efficiency of entomopathogenic nematodes (EPNs) as biological control agents of insect pests. We tested the hypothesis that EPNs associate spatially with a saprophytic fungus Fusarium solani in nature and contribute to both the fungal and nematode population growth, as found in previous laboratory-based studies that measured interactions at a fine spatial scale. Spatial Analysis by Distance Indices (SADIE) revealed highly significant spatial associations between F. solani and EPN communities comprising up to three species (Steinernema diaprepesi, Heterorhabditis indica, and Heterorhabditis zealandica), all of which were measured using quantitative real-time PCR during a 6-month citrus orchard survey. During three successive winter months, the EPNs aggregated in patches where F. solani aggregated during the previous month, although this behavior was not detected during the spring. The F. solani abundance in all plots was also related to that of total EPNs the previous month (one-month time lag; r = 0.17, P = 0.02) and may have resulted, at least in part, from the influence of soil moisture on both organisms. This study is the first report of quantitative relationships between F. solani and EPNs in the field. The results showed that a mutualistic relationship between this saprophytic fungus and EPNs reported previously, may have had measurable, but relatively minor effect on the spatial patterns of either organism at the landscape scale measured in this orchard.