An oil formulation of Neem Azadirachta indica was tested in four concentrations of 3.2% 4.8%, 6.4%, and 8.0% against larvae of Lymantria dispar in the laboratory. 1st instar larvae of the pest were fed on neem sprayed leaves of Prunus domestica. 100 percent mortality was observed in all neem test treatments within 21 days. In the highest dose (8%) maximum mortality was recorded after 72 hours which may be due to contact action of neem oil formulation. On the other hand, the lower doses caused slow mortality in the beginning and progressed steadily, owing to growth regulating behavior of neem. The neem formulation at 6.4% and 8% were phototoxic to Prunus domestica.