Extensive trials showed that a nematode of the family Neoaplectanidae and its associated bacterium did not act as a permanent or temporary biological control agent against the Colorado potato beetle, Leptinotarsa decemlineata (Say). These trials and the causes of the failure were investigated for a number of years at Belleville; the initial test was reported by Welch (1958). The nematode, commonly known as DD 136, was discovered in codling moth larvae by Dutky and Hough (1955). It is ingested with food by the host, passes through the gut wall into he haemocoele, and releases a bacterium that multiplies rapidly and kills the host. The nematode feelds on the body contents of the dead host, passes through several generations, and eventually leaves the host as an infective larva. Dutky, and we, found that many kinds of insects can be infected by the nematode in the laboratory. It may be reared in large numbers by infecting larvae of the greater wax moth, Galleria mellonella (L.). The larvae die within 36 hours at room temperature and are then placed on wet filter papers in a trap designed to recover the nematodes that emerge from the dead hosts. Storage of the nematodes in water at 45° F. for a month or more causes no decrease in vitality or infectivity, so that large numbers may be accumulated for field trials. These sturdy animals may be applied with a mechanical sprayer at pressures of 100 psi without injury.