SUMMARYThe rate at which a variety of nematode-trapping fungi captured nematode populations in agar cultures was determined. The number of da required to capture entire populations of nematodes under various sets of conditions was considered to reflect the overall predacity of each fungus tested. Parameters such as the age of the fungal colony, the number and type of nematode added, and the ability of the fungus to form traps within the agar all affected predacity, but the extent to which each condition did so varied with the fungus tested. The total number of da required by each fungus to capture nematode populations under all the conditions tested was termed the Predatory Index (PI). The larger the PI, the less predaceous the fungus was. Comparing PIs enabled the fungi to be ranked from effectively to poorly predaceous. PI values determined in vitro may be useful for predicting which fungi will be successful predators in soil because the PI indicates how well a fungus maintains its predacity under variable conditions.