The effects of lumicolchicine, colchicine, vinblastine and cold temperature on the pigment migration in melanophores and xanthophores of Fundulus heteroclitus and Oryzias latipes were examined by light and electron microscopy. Xanthophores of both species which contain numerous microfilaments and a poorly developed microtubule system were extremely sensitive to the alkaloids. Lumicolchicine and colchicine induced irreversible dispersion while vinblastine caused permanent aggregation of the pigments. Treatment in lumicolchicine or colchicine at 5 mM for 60 minutes did not disrupt microtubules of melanophores to an appreciable degree, however, melanosome aggregation was partially inhibited by these drug in Oryzias. When melanophores were kept in the cold in the presence of colchicine at 1 mM, almost all microtubules were disrupted and their repolymerization at room temperature was nearly completely inhibited by colchicine. These melanophores lacking in microtubules responded to epinephrine with slow aggregation. Vinblastine at 0.1 mM induced partial aggregation of melanosomes and disruption of microtubules but most melanophores were still able to respond with pigment migration. Vinblastine at 1 mM made all melanophores punctate and immobile. Large vinblastine-induced crystals were frequently seen in the dendritic processes. The results of the present investigation suggest that cytoplasmic microtubules in fish melanophores facilitate melanosome migration only in directional orientation and appear not be responsible for the motive force.