Total RNA was prepared from fat body of the silkworm, Bombyx mori, 9 hours after injected by E.coli K12D31. Single-strand cDNA was synthesized by reverse transcription (RT). Partial fragments of cecropin A cDNA were obtained, cloned and sequenced, by PCR technique with a pair of degenerate primers designed according to the amino acid sequence of CM4 in Bombyx mori and cecropin A in Hyalophora. These laid a foundation for further research on preparation of the probes of cecropin A for screening the silkworm cDNA library.