The nucleotide sequence of the structural gene for staphylococcal enterotoxin type C3 (entC3) was determined. This gene contains 798-base-pair open reading frame that encodes a protein of 266 amino acid residues. Sequence analysis suggests that staphylococcal enterotoxin type C3 is synthesized in a precursor form that is processed to yield a mature extracellular form of 238 amino acid residues (molecular weight, 27,438). The entC3 gene is closely related to the gene for staphylococcal enterotoxin type C1, with 98% nucleotide sequence identity. Sequence comparisons between the entC3, entC1, and entB genes suggest that an ancestral entC1-like gene was formed by recombination between the entC3 and entB genes.