Effect of lycorine on the growth and pigment formation in some marine algae. In this investigation the effect of lycorine on the growth and pigment synobasis in some red and green algae has been dettermined. It has been observed that lycorine inhibits mainly the growth of algae and, to some extent, their pigment content. Moreover, the different permeability of the apical segment to the alkaloid determines various inhibitory effects. Lycorine 2 × 10−5M prevents the growth of red algae at about the same rate as in green algae. It is particularly interesting to observe that the concentration 10−7M does not influence the growth ofPterocladiawhile inhibiting its pigment formation. Phycoerythrin has shown a sensitivity to lycorine higher than those of chlorophylls and carotenoids. It is established that carotenoid protect the cells of algae and higher plants, from lethal photoxidation. So, the inhibition of carotenoids formation is suggested as the primary effect of lycorine which is responsible for the highest reduction of phycoerythrin and also of the growth.