Phytophthora cactorum grew well in ammonium–salt media but not in nitrate. Conversely, P. megasperma grew well in nitrate but not in ammonium–salt media. P–parasitica and P. erythroseptica grew well in both, but better in that containing nitrate. All species grew better in the presence of organic nitrogen. Evidence is presented that iron, zinc, copper, and manganese are essential trace metals and that zinc is required in greater concentrations than iron. The optimum requirement for glucose or sucrose was found to be approximately 4%, and the residual sucrose was converted into reducing sugars by all species except P. cactorum. Approximately 50% of the residual sugar remained unchanged in the cultures of this species. The growth rate and sugar consumption increased as the thiamine level was increased from 0.2 to 0.8 mgm. per liter, but the coefficient of sugar utilization remained constant. On the other hand, the percentage of nitrogen in the mycelium decreased markedly as the thiamine content of the medium was increased. It is concluded that with media containing 1% alanine an accurate bio-assay of thiamine can be made over a concentration range of 0,2 to 0.6 mgm. per liter.