Cyclops vernalis was an avid predator of the preparasitic stage of the Mermithid nematode, Romanomermis culicivorax. Predation rate was dependent on the nutritional state of the copepods and the volume of water. In small volumes of water, 0.2 ml, satiated copepods killed preparasites within 100 min with a median predation time (PT50) of 41 min. Starved copepods killed preparasites within 70 min with a PT50 of 12 min. The rate of predation was inversely proportional to the water volume. In one liter of water, the infective ability of the preparasites for mosquito larvae was inversely proportional to predator density. A predator density of 53 copepods/liter reduced the number of infected mosquito larvae by 50%. These findings indicate that high densities of copepods in mosquito infested waters may interfere with mosquito control by R. culicivorax.