As reported in a previous article [Kataoka (1988a) Plant Cell Physiol. 29: 1323], growing apices of the xanthophycean coenocytic alga, Vaucheria terrestris, bends away from a unilateral blue light (BL) source, if they are simultaneously irradiated with strong background BL in a solution containing 1–4 mM Ca²⁺. Since the negative bending is a function of the product of the external Ca²⁺ concentration and the fluence rate of background BL, a BL-induced Ca²⁺-influx at the apex was hypothesized to be the cause of the phototropic inversion. The present report provides strong evidence for this hypothesis. Addition of the Ca²⁺ channel blockers, La³⁺, verapamil, nifedipine and nitrendipine to media containing 4 mM Ca²⁺ completely inhibited the phototropic inversion. By contrast, 1 μM A23187 (plus 4 mM Ca²⁺) not only enhanced the phototropic inversion under background BL, but also mimicked the background BL; i.e. it caused negative bending under safe red light. Inhibition of phototropic inversion by La³⁺ is also observed under conditions where the algae are irradiated with unilateral BL for 1 week. A BL-dependent Ca²⁺ influx and a consequential elevation of the cytoplasmic Ca²⁺-level in the apical growth region must be involved in the early steps of phototropic response. A BL-controlled opening of L-type Ca²⁺ channels is also suggested.