The cDNA sequence encoding the growth hormone (GH) I gene of goldfish (gf GH-I) was cloned into the pSXIVVI+ X3 baculovirus transfer vector. This transfer vector, without the initiation codon (ATG) and using a SynXIV promoter to activate transcription, was constructed for a baculovirus expression system. The recombinant transfer vector with the gf GH-I insert was used to produce the recombinant baculovirus TnNPV-SX+ gf GH-I 21a. Both in vivo and in vitro approaches were used to test the expression of growth hormone I gene using TnNPV-SX+ gf GH-I 21a. For in vivo studies, larvae of Trichoplusia ni were infected with the recombinant baculovirus. Four days later, growth hormone-like immunoreactivity was detected in the hemolymph of these infected larvae. For in vitro studies, insect Spodoptera frugiperda 9(Sf9) cells were infected with TnNPV-SX+ gf GH-I 21a. After incubation for at least 24 hours, growth hormone-like immunoreactivity was detected in Sf9 insect cells as well as in the culture medium. Western blotting analysis of larval hemolymph and Sf9 cell contents after viral infection revealed a protein band of 22.5 kDa immunoreactive to goldfish growth hormone antiserum. This is consistent with the predicted molecular weight deduced from the cDNA of goldfish growth hormone I gene. These results, taken together, suggest that the baculovirus expression system can be used to produce the recombinant growth hormone of a fish species.