Phytophthora cinnamomi and P. citricola failed to produce oospores on a 2-1 (g/liter) glucose-asparagine agar plus 10 mg/liter B-sitosterol (GAAs) with a C/N ratio of 3.8:1, but produced them on a 30-2 GAAs (C/N ratio 28.3:1); with asparagine at 2g/liter, the optimum glucose concentration for oospore production was 30-40 g/liter. The 30-2 GAAs was also favorable for oospore production by P. cambivora, P. capsici, P. cryptogea, P. palmivora, P. cactorum, P. heveae, and P. megasperma, whereas a 2-1 GAAs was unfavorable. With glucose at 5 g/liter and asparagine at 0.1 or 2.0 g/liter (C/N ratios 94:1 and 4.7:1), all the above species produced oospores abundantly at the 94:1 ratio except P. capsici and P. cryptogea; none produced oospores at the 4.7:1 ratio. Similar results were obtained with P. citricola, P. megasperma, P. cambivora, and P. cinnamomi when KNO3 was substituted for asparagine; however, P. heveae, P. cactorum, P. ilicis, P. capsici, and P. palmivora produced oospores equally abundantly at both concentrations of KNO3. Preliminary analysis indicates that NH4-N accumulates at the low C/N ratio with asparagine as the nitrogen source, and is associated with the rise in pH and inhibition of oospore formation.