Galls and regeneration in P. capillacea. – Galls arose from some cultured segments ofPterocladia capillaceain winter trough regeneration from cut-surfaces, especially at the proximal end. Bacteria were always present on the thallus and inside the old galls. They might be the agents of cecidogenesis, even though we never obtained galls in segments treated with these agents. We suggest that gall formation can be induced by bacteria only in winter, when the stimulus of natural differentiation appears to be very low.