The nutritional requirements of Aeropyrum pernix were studied to investigate the growth of a hyperthermophilic marine archaeon in a defined medium. Various nutrients including sugars, amino acids, and nucleoside bases were tested to determine their effects on cell growth. It was found that adenine and six amino acids (Arg, Ile, Leu, Lys, Met, Val) are essential for the growth of A. pernix. An artificial seawater-based medium, containing 11 amino acids, adenine, vitamins, KH2PO4, Na2S2O3, and trace minerals, enabled the growth of A. pernix at a cell density and specific growth rate similar to those obtained with complex media.