The nuclear polyhedrosis virus (NPV) of Porthetria dispar (gypsy moth) was isolated and purified in order to examine its protein components, which have been little studied in insect viruses. Virus particles were released by alkaline treatment and purified on a sucrose gradient, yielding seven virus fractions which (by electron microscopic examination) were shown to differ in the number of nucleocapsids per envelope. The most slowly sedimenting fraction contained only nucleocapsids and capsids. Analysis of each fraction by sodium dodecyl sulfate polyacrylamide gel electrophoresis showed that the virus particles contain 14 proteins; 11 are seen in the nucleocapsid. Three proteins are seen in the polyhedral protein fraction.