Red algae are widely known to produce floridean starch but it remains unclear whether the molecular structure of this algal polyglucan is distinct from that of the starch synthesized by vascular plants and green algae. The present study shows that the unicellular species Porphyridium purpureum R-1 (order Porphyridiales, class Bangiophyceae) produces both amylopectin- type and amylose-type alpha-polyglucans. In contrast, Cyanidium caldarium (order Porphyridiales, class Bangiophyceae) synthesizes glycogentype polyglucan, but not amylose. Detailed analysis of alpha-1,4-chain length distribution of R purpureum polyglucan suggests that the branched polyglucan has a less ordered structure, referred to as semiamylopectin, as compared with amylopectin of rice endosperm having a tandem-cluster structure. The P. purpureum linear amylose-type polyglucan, which has a lambda(max) of 630 nm typical of amylose-iodine complex and is resistant to Pseudomonas isoamylase digestion, accounts for less than 10% of the total polyglucans. We produced and isolated a cDNA encoding a granule-bound starch synthase (GBSS)type protein of R purpureum, which is probably the approximately 60-kDa protein bound tightly to the starch granules, resembling the amylose- synthesizing GBSS protein of green plants. The present investigation indicates that the class Bangiophyceae includes species producing both semi-amylopectin and amylose, and species producing glycogen alone.