We report measurements of the linear viscoelastic modulus of collagen gels. Collagen gels are cross-linked networks of bundles formed from the association of single filaments. Their structure is heterogeneous at microscopic length scales, and measurements of their viscoelasticity at a larger scale are required. We use a technique that we recently developed based on the measurement of surface thermal fluctuations and called surface fluctuation specular reflection [Pottier et al., Soft Matter 7, 7843–7850 (2011)] to characterize the linear viscoelastic properties of collagen gels for frequencies ranging from 1 Hz to 30 kHz. These properties are determined at a mesoscopic scale (∼100 μm) at which the gels are homogeneous. We further study the influence of both collagen origin and concentration in the light of previous findings on these systems.