Three isoforms of laminaribiose phosphorylase, F0, F1, and F2, were purified to an electrophoretically homogeneous state from a cell free extract of Euglena gracilis Z (IAM E-6). F1 and F2 were the major components. The three enzymes showed very similar properties except their isoelectric points. They could also use laminaribiose as the glucosyl acceptor instead of glucose. The isoforms the same molecular mass of 120 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (PAGE) or 200 kDa by the gel filtration method, suggesting that they have a dimer structure. They could not be distinguished on the Ouchterlony's double diffusion test with mouse antiserum against F1 or F2. The substrate specificities of F1 and F2 were determined to be essentially the same. Both of the reaction mechanisms of F1 and F2 were determined to be ordered bi-bi mechanisms, as in the case of cellobiose phosphorylase. A competitive substrate inhibition was observed in their synthetic reaction. Two other strains, E. gracilis var. bacillaris (IAM E-2) and E. gracilis (IAM E-3), had only one laminaribiose phosphorylase, which corresponded to F0 on native PAGE.