The life cycle of a new marine Phycomycete, Thraustochytrium aureum, is described based on its development on pollen in sea water. Comparisons are made with the amoeboid habit of the fungus on nutrient agar and an hypothesis as to the cause of the phenomenon is discussed. Inoculation studies suggest that T. aureum may be primarily a saprophyte of vascular plant debris. The fungus is obligately marine and apparently euryhaline. Maximal growth occurs with 2.0% NaCl not replaceable by KCl. The omission of KCl from media does not affect yields. The salinity optimum does not vary with temperature and the organism grows best at 12–25°C. It can utilize the nitrogen in (NH4)2SO4 but optimal harvests are obtained with glutamate as the source of that element and with carbon supplied as glucose, maltose, or soluble starch.