AbstractSex cell adhesion in isogamous chlamydomonads is caused by a complementarity between sex‐specific mating type substances, glycoproteins anchored in the flagella membrane of (+) and (\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop - \limits^. $\end{document}) gametes. The systems of mating type substances are species‐specific and condition, by their individuality, gametic incompatibility between species. The adhesion systems of several species share one common feature: the attainment of the agglutination capacity is sensitive to tunicamycin, but in one sex only. The effect is interpreted as due to the interference of tunicamycin with the synthesis of the mating type substances by blocking of their glycosylation in one but not in the other sex. It is postulated that the tunicamycin‐sensitive gametic adhesiveness depends, within the mating‐type‐specific glycoprotein, on an N‐glycosidically bound ligand of carbohydrate nature. A concept on the origin of sibling species by mutative modulations within the proper ligands of the glycoproteinaceous mating type substances i