We have analysed TFP3 transposable elements from five independently isolated FP mutants of the Autographa californica nuclear polyhderosis virus (AcMNPV). We also analysed genomic copies of TFP3 elements amplified from the DNAs of the Trichoplusia ni cell line (TN-368) and T. ni larvae using the polymerase chain reaction (PCR). The sequences of all the newly isolated TFP3 elements closely resemble the previously described TFP3/1 element. Each of the transposons isolated from the virus mutants duplicated a TTAA tetranucleotide target site upon insertion into the viral genome. Four of these TFP3 elements transposed into three different 'TTAA' target sites within the 25 K gene (FP locus, map units 36-37 of AcMNPV). The fifth TFP3 element inserted at a 'TTAA' site within the AcMNPV Hin dIII-E fragment. One genomic TFP3 element, amplified from the TN-368 cell line DNA by an inverse PCR method, duplicated a 'TTAA' tetranucleotide target site that is present only once in the homologous larval DNA sequence. These data suggest that mobilization of TFP3 into both viral and cellular sites is identical in specificity and mechanism.