A homogeneous agaran fraction from Palisada flagellifera ( Laurencia complex, Rhodomelaceae, Ceramiales) was obtained by aqueous room-temperature extraction, followed by ion-exchange chromatography. This galactan presents a highly complex structure with at least 18 different types of derivatives. The A units were found mostly pyruvylated, 2-sulfated (∼34%), and 6-methylated (∼34%), with the latter partially 2- and 2,4-sulfated. Minor amounts of β- d-galactopyranosyl units 2-, 6- and 2,6-sulfated, 6-glycosylated, and non-substituted are also present. The B-units are l-sugars composed predominantly of their cyclized derivatives, 3,6-anhydrogalactose and 3,6-anhydro-2- O-methylgalactose (∼56%). The former are linked to β- d-galactosyl (6-methyl) (6-glycosylated) units, as well as to 4,6- O-(1-carboxyethylidene)-β- d-galactose 2-sulfate in the proportion of 3:1.8, respectively. A significant amount (∼18%) of the α- l-galactopyranosyl units are linked to pyruvylated β- d-galactose 2-sulfate residues. An important part of the B-units (20%) is represented by α- l-galactose 6-sulfate substituted on C-3 by xylosyl, galactosyl and/or 2,3-di- O-methylgalactose units or sulfate groups that preclude their cyclization to 3,6-anhydrogalactosyl derivative. The precursor units are present in relatively low percentages. Kinetic studies suggest that in P. flagellifera agaran the cyclizable units are linked to 6- O-methyl-β- d-galactosyl and/or β- d-galactosyl units (6-glycosylated). The structural complexity of this polysaccharide is increased by the presence of 2- and 3,6-sulfated α- l-galactoses, with the latter additionally 2-O-methylated. Therefore, the major subfraction obtained from the cold extract contains structurally complex sulfated, methylated, and pyruvylated agaran.