Dead larvae of Pieris rapae L., showing symptoms of nucleoplyhedrovirus (NPV), were collected from cabbage and cauliflower fields. Microscopical examination of corpus body fluid exhibited polyhedral occlusion bodies (POBs). Larvae of the 4th instar (L4) were infected with the virus suspension at the concentrations of 4x104, 4x106, 4x108, and 4x1010 POBs/ml. Bioassay tests revealed that egg incubation period of P. rapae, reared on a semi-synthetic diet, was 4.29±0.24 days, total larval period was 16 ± 1.67 days and pupal period was 5 ± 0.6 days). Seven days after treatment of larvae with POBs, larval mortality of L2 increased by increase of POBs concentration to 30, 42, 58 and 62% and reached 60, 70, 82 and 100% on the 8th day for the successive respective POBs concentrations. Treating larvae of L4 with the same respective concentrations, showed death among treated larvae one day earlier than in case of L2. Mortality values of treated L4 recorded 60, 66, 72 and 84% on the 7th day after treatment. These values increased to 84 and 92% for the concentrations 4x104, 4x106 and 100% for the concentrations 4x108, and 4x1010 POBs/ml, on the 8th day, respectively.