The cell wall of the red microalgae Porphyridium sp. (UTEX 637) comprises a complex amorphous polysaccharide (6–7 × 106 Da). The polysaccharide is made up of xylose, glucose, and galactose as the main sugars, as well as some minor sugars, protein, sulfate, and glucuronic acid, the latter two conferring a negative charge on the polysaccharide. In this study, we used synchronized cultures as one of the ways of unraveling the mechanism of biosynthesis of this complex polysaccharide by following cell-wall formation during the cell cycle. Synchronization of Porphyridium sp. was achieved with an alternating light:dark regime of 12:12 h LD and dilution of the culture at the end of the cycle. Under these conditions, cell duplication occurred between the 12th and 14th hours of the cycle. The following order of building toward formation of the final polysaccharide appeared to take place: Intermediate polysaccharides with molecular masses ranging from 0.5 × 106 to 2 × 106 Da appeared in succession during hours 2–6 of the cycle, and the full-sized polysaccharide was detected by the 8th hour. At the beginning of the cycle, xylose was the predominant sugar. Sulfur peaked at hours 2–4; glucose, galactose, and glucuronic acid at hours 8–12; and the minor sugars at hours 12–14. Upon incubation of low molecular mass polymer (0.5 × 106 Da) collected from the 4th hour with cellular crude extract from cells of the 6th hour of the cycle, two intermediates were formed (0.8 × 106 Da and 2 × 106 Da). We suggest that the 0.5 × 106 Da polymer intermediate, which is composed mainly of xylose, is the first polymer secreted into the medium, where it is further polymerized enzymatically to produce the 2 × 106 Da polymer via an intermediate 0.8 × 106 Da polymer. Later, the full-size polysaccharide is produced.