The polyhedrin gene (polh) was identified from the Trichoplusia ni (Tni) single capsid nuclear polyhedrosis virus (SNPV). An EcoRI fragment containing the truncated polyhedrin gene was detected by hybridization with an AcMNPV expression vector probe; the remaining portion of the gene was amplified by reverse PCR. An open reading frame (ORF) of 741 nucleotides (nt), encoding a putative protein of 246 amino acids (a.a) with Mr 28,780 Da was identified. The 5'-noncoding region contained the putative late (TAAG) transcription initiation motif. The 3' end, downstream of the translation stop codon, lacked an obvious putative poly (A) signal. Nucleotide and amino acid homology are greater than 80% to that of Mamestra brassicae polyhedrin sequences. Results suggest that T. niSNPV is a member of the group II nuclear polyhedrosis viruses.