Effects of salinity and pH were assessed on the infective capacity of parasitic nematodes of mosquitoes Romanomermis culicivorax Ross & Smith and Romanomermis iyengari (Welch), on larvae of southern house mosquito, Culex quinquefasciatus Say. Thirteen pH levels (3.6, 3.8, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.2, and 8.4) were evaluated. Six salinity levels (NaCl concentrations) were evaluated, with values of 0.01 M (584 mg/L), 0.02 M (1,169 mg/L), 0.03 M (1,753) mg/L), 0.04 M (2,338 mg/L), 0.05 M (2,922 mg/L), and a 0.00 M check, with three replications. Percentage of infection (of parasitized larvae) and infestation intensity (average nematodes parasitizing a mosquito larva) were recorded for both factors. To determine effects of pH and salinity on percentage of infection and infestation intensity, a sample (n = 20, IV instar) of larvae surviving at high salinity levels was extracted 5 days after application. The greatest percentage of infection statistically significant in R. iyengari was 75 to 91.7 for pH 5.0 to 7.5, and 86.7 to 100 for R. culicivorax between pH 6.0 and 7.0. The greatest infestation intensity statistically significant in R. iyengari was 4.42 to 8.45 between pH 4.5 and 8.0. The greatest levels of infestation statistically significant in R. culicivorax were between pH 5.0 and 7.5 with 3.07 and 3.63 nematodes per mosquito larva.