Previously, we have shown that the monomeric-sugar composition of cell-surface-associated glycoconjugates of two strains of Chlamydomonas eugametos, of different mating type, differs strikingly (Gerwig et al. 1984, Carbohydr. Res. 127, 245-251). Besides the common occurrence of various pentoses and hexoses, the glycoconjugates of one strain contain 4-O-methyl xylose, a 2-O-methyl pentose (probably 2-O-methyl arabinose) and 3-O-methyl galactose, whereas those of the other strain contain 6-O-methyl mannose and 3-O-methyl glucose. In order to investigate whether these differences are relevant to the mating process of this organism, the sugar composition of the sexual progeny of these strains was analyzed. The ability to produce 4-O-methyl xylose, 2-O-methyl pentose and 3-O-methyl galactose on the one hand, and the ability to produce 6-O-methyl mannose and 3-O-methyl glucose on the other hand, appear to be genetically linked. However, the ability to produce either set of O-methyl sugars was inherited independently of mating type. O-Methylated sugars do not occur in the cell wall of C. eugametos, or in the cell-free medium, but only in surface-membrane-associated glycoconjugates, extractable with salt or detergent solutions.