This study was undertaken to investigate the mineral nutrition of aquatic Hyphomycetes by determining the mineral requirements of several species. On the basis of rapidity and renroducibility of growth, four species were selected, These were Heliscus lugdunensis Webster (Nectria lugdunensis webster), Flagellospora penicillioides Ingold (Nectria penicillioides Ranzoni), Clavariopsis aquatica DeWildeman, and Lunulospora curvula Ingold. These were grown in liquid culture on a rotary shaker at constant temperature. The containers and other equipment were cleaned by acid washing. When necessary to establish a nutritional reguirement for an element, the nutrient salts were purified by the phosphate adsorption and dithizone methods. In addition to nutrient salts and purified water, the media contained only glucose. Since all the species except Clavariopsis aquatica DeWildeman grew well for many transfers and approximately two years on this completely defined medium, without vitamins or amino acids being provided, it is concluded that they do not require these nutrients. Whether an element was required, and in what concentration, was decided on the basis of the amounts of growth obtained with various concentrations of the element in the initial media. Growth was measured as the dry weight of the mycelium from 100 ml of medium and the values from four replicates were averaged. Phosphorus, calcium, nitrogen, zinc, iron, magnesium, and sulfur requirements were demonstrated for all four species. Efforts to demonstrate requirements for potassium were unsuccessful. Manganese toxicity was demonstrated for Clavariopsis aquatica DeWildeman and a manganese requirement was suggested for Flagellospora penicillioides Ingold (Nectria penicillioides Ranzoni). The results suggested a molybdenum requirement for Heliscus lugdunensis Webster (Neetria lugdunensis Webster) and Lunulospora curvula Ingold, a boron requirement for H. lugdunensis and toxicity for C. aquatica, and a copper requirement for F. penicillioides. Approximately 0.50, 0.25, 0.50-1.00, and 0,.05-0.10 milli-moles per liter of phosphate were required to obtain maximum dry weight yields of Heliscus lugdunensis Webster (Nectria lugdunensis Webster), Flagellospora penicillioides Ingold (Nectria penicillioides Ranzoni), Clavariopsis aguatica DeWildeman, and Lunulospora curvula Ingold respectively. Calcium concentrations of 7.5, 50, 7.5, and 25 milli-moles per liter supported maximum growth of these species in the same order. All four were able to utilize ammonium nitrogen, nitrate nitrogen, or ammonium nitrate to produce about the same amounts of growth. C. aquatica was apparently able to use nitrate nitrogen only in the presence of an unidentified nutrient and this is interpreted as indicating a role for that nutrient in the nitrate metabolism of this species. Fifteen milli-equivalents per liter of ammonium nitrate, ammonium, or nitrate nitrogen was adequate to obtain maximum dry weight yields of H. lugdunensis. Five milli-equivalents per liter of ammonium nitrate, 15 of ammonium, and 35 of nitrate nitrogen supported maximum growth of F. penicillioides. As little as 5 milli-equivalents per liter of ammonium nitrate or ammonium nitrogen was optimal for C. aguatica and 5 milli-equivalents per liter of any of the three nitrogen sources was optimal for L. curvula. The optimal nitrate concentration for C. aquatica was not determined. There is an obvious advantage to a single medium on which a great number and wide variety of species may be grown. The author is of the opinion that a far greater advantage lies in the use of a completely defined medium on which one or more organisms may be reproducibly grown, Such media are suggested and their employment is strongly urged. Only in this manner can considerable morphological and physiological variations be avoided. / Ph. D.