In flooded rice fields a decline in total nematode populations began shortly after the onset of soil anaerobiosis and was correlated with a sustained increase in concentration of molecular hydrogen sulfide in the soil-water phase. Laboratory tests showed that hydrogen sulfide at concentrations found in flooded fields killed 100 percent of nematodes in 5 to 10 days. The effect of hydrogen sulfide and its pattern of occurrence in the soils of water-saturated rice fields suggest that this compound can be a significant factor in the etiology and control of certain plant diseases.