SUMMARYMinimum phosphorus requirements of 2 marine diatoms,Cylindrotheca (Nitzschia) closteriumandCyclotella nana, were measured in axenic steady state chemostat culture.C. closteriumandC. nanarequired 1.055 and 1.045 × 10−15g‐at P cell−1generation−1, respectively, with an average cell doubling time of 0.20 divisions/day. The algae were cultured at 15.5 C and exposed to 5380 lux on a light‐dark cycle of 19 hr‐5 hr. The phosphorus requirement per unit of cell volume was 0.0135 and 0.0129 × 10−15g‐at P/μ3of cell volume forC. closteriumandC.