Rhizoids of Fucus spiralis were cultivated axenically in the artificial seawater ASP6 F2. Experiments were made to increase the filamental growth as well as to induce adventive primordia (plantlets). Additions of such carbon compounds as glucose, acetate, and formate had no favourable effects even in concentrations as low as 1∙10−4 M. Mannitol killed the rhizoids in higher concentrations and inhibited growth even in a concentration as low as 1∙10−5 M. Higher concentrations of glycerol also inhibited growth, but 1∙10−4 M was an exception as it initiated plantlets. Many simple phenolic compounds induced plantlets. Among the most active substances were phenylacetic acid, p-hydroxyphenylacetic acid, 3,4-dihydroxybenzoic acid, o-hydroxybenzoic acid, and o-acetoxybenzoic acid, with optimal effects in the concentration range of 1∙10−7 to 1∙10−6 M. β-Indolylacetic acid strongly influenced the dry weight as well as plantlet formation at concentrations of 1∙10−8 to 1∙10−7 M, with 1∙10−8 M favouring plantlet induction. It is obvious that β-indolylacetic acid plays an important role in the earlier stages of the development of Fucus.