We describe resonant excitation of a plasmon by the Gaussian-like leaky mode of an effectively single mode photonic crystal (PC) waveguide. Plasmon is phase matched by design with a waveguide mode, and travels in a metallic layer on the top of a PC waveguide. We observe that small changes in the ambient refractive index just outside the metal film lead to strong variations in the losses of a waveguide core mode, making such a device a good candidate for compact sensor applications. Using low refractive index core PC waveguides makes phase matching between the plasmon and PC core mode relatively easy to enforce at any wavelength of operation as modal effective refractive index in such waveguides can be readily varied in a wide range.