When polyhedra ofAutographa californicapassaged several times inSpodoptera exiguawere passaged inTrichoplusia ni, the size of the polyhedra increased considerably. In addition, these polyhedra contained more virions per cross section of polyhedral inclusion body (PIB), contained a greater mean and mode of nucleocapsids per virion, and were more virulent to neonateT. ni. The polyhedra ofT. niafter several passages inT. niwere not altered in size when passaged inS. exigua. However, these polyhedra had more virions per cross section of PIB, had a greater mean and mode of nucleocapsids per virion, and were more virulent to neonateT. nithan were PIBs ofT. niproduced inT. nilarvae.