The spatial distribution of the waterscorpion Ranatra nigra Herrich-Schaeffer (Hemiptera: Nepidae) in the Mobile/Tensaw Delta, Mobile, Alabama, was examined. Twenty-seven sites were sampled once in the fall (10–16 October 2001) and again the following spring (22 May-5 June 2001). A principal component analysis in combination with logistic (presence/absence) and stepwise (abundance) regression was used to associate the distribution of R. nigra with select abiotic variables (pH, conductivity, temperature, oxygen, stream width, GPS coordinates). Both the occurrence (presence/absence) and abundance of R. nigra were significantly (P < 0.001) associated with GPS coordinates, i.e., it was most common closer to the Mobile Bay than further into the Delta. In addition, R. nigra was more likely to occur in smaller, cooler streams, with lower oxygen and pH levels and higher conductivity. The prevalence (percentage of infected host) and abundance (number of mites/infected host) of the parasitic mite Hydrachna magniscutata (sensu latu) Marshall on R. nigra were also examined. The proportion of R. nigra with mites was highest in the fall and lowest in the spring (P < 0.001), but the number of mites per host did not change (P > 0.05) with time.