A pond was treated 8 times (September, 1973--November, 1975) with material derived from a laboratory culture of the Mermithid nematode, Diximermis peterseni. The inundative effect of these treatments against Anopheles crucians and Ae. quadrimaculatus was variable. The inoculative effect of the releases was followed for almost 3 years (April, 1974-June, 1976). Levels of parasitism increased each year from 15 to 37 to 42%. Infection levels averaged 20% during the warmer months (April through September) as opposed to 32% during the cooler months (October through March).