Melanopsin is a novel opsin expressed in a small sub-set (~1%) of directly photosensitive retinal ganglion cells which project to brain areas involved in various non-image forming functions of the eye, including the pupil light reflex, circadian photoentrainment and light-suppression of the synthesis of the pineal gland hormone, melatonin. The mechanisms by which melanopsin transduces photic information remain poorly understood (Peirson & Foster, 2006). In the present study a clonal X. laevis melanophore cell line which endogenously expresses melanopsin was used to investigate the phototransduction cascade activated by light. Exposure of melanophores to white light (100 μW/cm²) triggered a dramatic dispersion of pigment granules throughout the cell, but did not alter intracellular cyclic GMP concentration. Also photodispersion of pigment was not blocked by a cyclic GMP analog (8Br-cGMP, 100 μM) or a cyclic GMP phosphodiesterase inhibitor (zaprinast, 100 μM) suggesting that the classical rod and cone opsin phototransduction cascade is not activated by melanopsin. Furthermore, in melanophores, RT-PCR did not detect mRNA for transducin α-subunit or arrestin, key components of this pathway. As melanopsin has high homology with invertebrate opsins, the G_q-phospholipase C-protein kinase C (PKC) phototransduction pathway utilised by invertebrate rhabdomeric photoreceptors was examined. A PKC inhibitor (Ro31-8220, 100 μM) did not alter photodispersion but did effectively block dispersion in response to a PKC activator (PDBu, 100 nM), indicating that this pathway is not involved in the response to light. In contrast, photodispersion was significantly inhibited in melanophores transfected with a G_sα dominant negative mutant, and light exposure produced a rapid, large (~2-fold), sustained (≥ 60 min) and highly significant (p<0.001) increase in melanophore cyclic AMP concentration. A role for protein kinase A (PKA) was indicated as microinjection of PKA inhibitors (PKI 6-22 amide or Rp-8Br-cAMPS) completely blocked photodispersion: increase in pigmented area in control melanophores, 124 ± 46%; PKI 6-22 amide treated melanophores, -6 ± 8% (n=4-6, p<0.05, Student’s t test). These experiments show that the signalling mechanism activated by melanopsin in melanophores is neither of the previously characterised mechanisms of phototransduction found in vertebrates or invertebrates. Rather, in these cells light activates G_s, which increases adenylate cyclase elevating intracellular cyclic AMP leading to activation of PKA. As PKA is known to be physically associated with pigment granules in a complex with the molecular motors which drive translocation (Kashina et al. 2004), this may provide the basis for light regulation of pigment distribution in melanophores.