In this work, resonance phenomena in a negative photonic crystal are experimentally detected and discussed. Localized surface modes and guided mode resonances appear in the reflection spectrum of a photonic crystal slab interacting with external infrared radiation and can be connected with the negative refractive index of the sample . These phenomena can provide an efficient way to confine the radiation into the structure, with an high field enhancement and a strong sensibility of the resonance position to the refractive index variations.