Apical tips of Fucus vesiculosus L. were grown in sterile sea water. The apical cell region was excised surgically and this resulted in a decreased rate of growth of the segment and the regeneration of new branches. Treatment of cultured tips, whose apical cell region had been excised, with four auxins and two anti-auxins revealed a striking effect. The latter treatment induced up to three or four times the number of branches regenerated in control experiments. Extraction and chromatographic procedures carried out on apical tips from November to January showed that a mixture of stimulatory and inhibitor)’growth substances was present. It is concluded that growth regulators play a part in determining apical dominance in Fucus.