We characterize a new Lepidopteran transposon associated with FP mutations of baculoviruses. This transposon, designated hitchhiker, is 579 bp long with 39-bp imperfect inverted terminal repeals. hitchhiker inserts with extreme specificity for a single trinucleotide target site, TTA, within the 25K gene. This transposon is the most frequently identified insertion in serial passage baculovirus mutants isolated from TN-368 cells. (C) 1996 Academic Press, Inc.