Lagenidium callinectes Couch and Haliphthoros milfordensis Vishniac. facultative parasites of marine crustaceans, were subjected to a detailed analysis of their physiological and nutritional requirements. Twelve isolates, six of each species. were used in this study. Temperature, salinity, and pH ranges of growth were determined. Vitamins B 1 and B 12 were required by some isolates of L. callinectes, but not by H. milfordensis. A variety of amino acids were utilized as nitrogen sources. Urea was used also by isolates of both species, while nitrate was used by only H. milfordensis. The number of carbon sources utilized was rather limited. Some sugars, such as fructose, glucose, mannose, cellobiose (poorly), maltose and trehalose, and, of the polysaccharides tested, only soluble reserve polysaccharides, such as starch, dextran, laminarin, and glycogen, were used in nutrition. The isolates of H. milfordensis were more versatile but more uniform re'garding their nutritional requirements than the isolates of L. eallil/eetes. The latter fall into two groups, two rather fast growing, versatile isolates not requiring NaCI, and four slower growing obligately marine strains, with a more limited range of utilizeable substrates.