The requirements for soluble protein concentration in sunflower seed extract (SFE) cultures were similar for 3 isolates of Lagenidium giganteum from North Carolina, Louisiana and California. Sunflower seed extract agar containing soluble protein in the range of 1.0 to 4.0 mg/ml was satisfactory for high levels of zoospore production. It is recommended that the protein concentration be 2.0 mg/ml for both SFE agar and liquid SFE culturing media, and the protein assay be with the Bradford method using bovine serum albumin as the standard.