cDNA sequences encoding the 42 kDa glycoprotein elicitor from the oomycete, Phytophthora megasperma, that induces the defense response in parsley have been cloned and sequenced. The 5' end of the mRNA matches a consensus derived from sequences surrounding the transcription initiation sites of seven other oomycete genes. The major transcript of 1802 nucleotides contains a 529-codon open reading frame, which was predicted to encode a 57 kDa precursor protein. On the basis of peptide sequencing, the N-terminus of the mature protein is at position 163, suggesting that proteolytic processing events, in addition to signal peptide cleavage, generate the protein purified from the fungal culture filtrate. Expression studies in Escherichia coli with the cDNA as well as smaller subfragments demonstrated that a region of 47 amino acids located in the C-terminal third of the protein was sufficient to confer elicitor activity. The gene encoding the elicitor was found to be a member of a multigene family in P. megasperma. Homologous families of differing sizes were found in all eight other Phytophthora species tested, but not in other filamentous fungi including other Oomycetes. No significant similarity of the elicitor preprotein to sequences present in the databases has yet been detected.