Vitamin B₁₂ content of an edible cyanobacterium, Nostochopsis sp. was determined to be 140.6±16.2 μg/100 g dry weight by a microbiological method. To evaluate whether the Nostochopsis cells contain vitamin B₁₂ or inactive corrinoid compounds, corrinoid compounds were purified from the cells and then identified as pseudovitamin B₁₂ (97.4±11.8 μg/100 g dry weight) and vitamin B₁₂ (43.2±6.0 μg/100 g dry weight) on the basis of silica gel 60 TLC bioautograms and LC/ESI-MS/MS chromatograms. Vitamin B₁₂ content was significantly increased in the Nostochopsis cells (254.8±17.6 μg/100 g dry weight) grown in the vitamin B₁₂-supplemented medium.