A liquid synthetic medium is described in which Vibrio costicolus grows well in 24 hours. The medium contains glucose, L-cystine, L-glutamic acid, L-arginine, DL-valine, DL-isoleucine, potassi~~m phosphate buffer, magnesium sulphate, sodium nitrate, and sodium chloride. The addition of 10 vitamins, 3 bases, 14 amino acids, and 5 salts did not improve the medium. No growth was obtained if cystine, glucose, or sodium chloride was omitted from the medium. The remaining constituents of the simplified medium were not essential but were found necessary for maximum growth. An amino acid antagonism was observed between valine and isoleucine. When added alone, either amino acid depressed the growth response, but when added together they were stimulatory.