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Four aquatic fungi - Apodachlya brachynema and A. minima (Leptomitales), Aphanomyces laevis (Saprolegniales), and Pythium ultimum (Peronosporales) - were tested for growth in synthetic media containing one of a variety of carbon sources. Apodachlya brachynema readily utilized five amino acids - alanine, glutamate, aspartate, proline and leucine - as well as glucose and acetate. Growth on sucrose as a carbon source was slight. Apodachlya minima differed from A. brachynema in that it could not utilize proline and leucine. Aphanomyces laevis grew well on only three of the substrates tested - glucose, alanine and glutamate. Pythium ultimum utilized glucose, sucrose, maltose, cellobiose, alanine, glutamate, aspartate, proline asparagine, ornithine, and serine, but not eight other amino acids. All of these fungi hydrolyzed gelatin. Radioactively labeled carbon dioxide was released during incubation of Aphanomyces laevis in media containing labeled leucine, proline, or phenylalanine. These data provide evidence of some catabolism of the three substrates although none of these substrates can support the growth of Aphanomyces laevis as a sole source of carbon and nitrogen. [ABSTRACT FROM AUTHOR]