The production of toxins by Aeromonas species was examined by the suckling mouse test, the hemolysin test, and the enzyme-linked immunosorbent assay with anticholera enterotoxin. A factor that was immunologically related to cholera enterotoxin was produced by 5 of 14 strains of Aeromonas hydrophila and 4 of 15 strains of Aeromonas sobria. Analysis by these assays and by a test for heat stability suggested that the factor differed from hemolysin and from toxin that was active in the suckling mouse test.