SUMMARYStudies of nutrition and growth were carried out with two isolates of the marine fungusZalerion maritimum.Experiments showed no difference in growth between flasks which received 0.75, 1.5, 3.0, and 4.5 mg/ml inoculum. One isolate grew on nutrient solution which contained thiamine, alone or in combination with other vitamins; the other isolate grew on nutrient solution which included thiamine and/or biotin. Experiments with reinoculated medium showed that growth was reduced in batch culture by the lowpH conditions produced during growth in a nutrient solution containing ammonium. MES ([2-N-morpholino] ethanesulfonic acid) buffer maintained thepH of the nutrient solution closer to the original value of 7.5 than did THAM (tris [hydroxymethyl] aminomethane) buffer. Chemical analyses of the nutrient solution during growth ofZ. maritimumshowed that ammonium was taken up to the complete exclusion of nitrate. After the 2-wk incubation period, 20% of the glucose and all of the phosphate had been removed from the nutrient solution. We suggest that standard media for the cultivation of marine fungi be adjusted to decrease the concentration of glucose, and increase the concentration of phosphate. We also suggest that MES replace THAM as a buffer in ammonium-containing media.