We propose a longitudinal spin-Seebeck effect (SSE), in which a magnon-induced spin current is injected parallel to a temperature gradient from a ferromagnet into an attached paramagnetic metal. The longitudinal SSE is measured in a simple and versatile system composed of a ferrimagnetic insulator Y3 Fe5 O12 slab and a Pt film by means of the inverse spin-Hall effect. The experimental results highlight the intriguing character of the longitudinal SSE due to its own geometric configuration. © 2010 American Institute of Physics.