Steinernema scarabaei, isolated from epizootics in larval populations of Japanese beetle and oriental beetle in New Jersey, has shown exceptional efficacy for curative control and long-term suppression of white grubs, the root-feeding larvae of scarab beetles. To improve predictability of S. scarabaei applications we studied the effect of 7 soil/substrate types and soil moisture levels on its efficacy and persistence. In laboratory and greenhouse experiments, S. scarabaei infectivity and efficacy was not strongly affected by substrate type but tended to be the highest in a loamy sand, did not differ significantly among sandy loam, loam, silt loam, and clay loam, and tended to be the lowest in a highly acidic sand (pH 3.9) and a typical potting mix (69% OM). The effect of soil moisture on S. scarabaei infectivity was studied in the laboratory in 3 selected soils. Moisture levels ranged from saturated to very dry (-1 to -3,000 kPa water potential) (gravimetric: loamy sand, 16 - 2.3%; sandy loam, 22 – 4.2%; silt loam, 30 – 6%). In loamy sand, there was no strong effect of soil moisture. Even at –3,000 kPa 70% of the grubs were killed. In sandy loam and silt loam, the mortality and infectivity were highest at –10 and –100 kPa, lower at –1 and –1,000 kPa, and the lowest levels at –3,000 kPa. This effect was stronger in silt loam than in sandy loam.