A defined medium was developed for a bacterized Gladophora glomerata from Lake Michigan yielding 0.8 g dry weight/m(2) /20 days and a doubling time of 1.2 days. Best growth was at 25 C under continuous aeration and 200-500 ft-c light. Aeration with 2% CO2 , with or without THIS buffer, inhibited growth. Calcium nitrate, urea, and NaNO3 were all utilized as nitrogen sources; however, more NO3 nitrogen was required than urea or Ca nitrate N for maximum yields. Germination of zoospores through zoosporangial walls was observed.