Spontaneous mutants of insect nuclear polyhedrosis viruses of Autographa californica (AcMNPV) and Galleria mellonella (GmMNPV) were analyzed. These mutants produce few polyhedra in infected cells and have insertions of host cell DNA. All the insertions mapped to two adjacent sites in the genome. The junctions between two host insertions and the viral DNA were sequenced. One of the insertions contained a perfect 7-bp inverted terminal repeat, and had caused a direct duplication of 4 bp of viral DNA at the point of insertion. Therefore, this insertion sequence has properties of a transposon of the host cell Trichoplusia ni.