The new nonlinear crystal for the mid-IR CdSiP2 was discovered only very recently but the interest in this chalcopyrite is enormous because it possesses most of the attractive properties of the related ZnGeP2 but allows in addition pumping at 1064 nm without two-photon absorption and uncritical phase-matching for 6 μm generation with maximized effective nonlinearity. The last feature is due to the fact that this crystal is negative uniaxial in contrast to ZnGeP2 which shows positive birefringence. We now measured its nonlinear coefficient using SHG of femtosecond pulses generated near 4.6 μm from a seeded KNbO3 optical parametric amplifier. The SHG efficiency was compared for uncoated samples of CdSiP2 and ZnGeP2, both ≈0.5 mm thick, in the low conversion limit (<10% internal conversion efficiency) which justifies the use of the plane wave approximation. Taking into account the experimentally determined phase-matching angles for type-I SHG (oo-e type in CdSiP2 and ee-o type in ZnGeP2), which were in good agreement with the existing Sellmeier approximations, we arrived at d36(CdSiP2)~d36(ZnGeP2) which is rather unexpected having in mind the larger band-gap of CdSiP2. The reliability of the measurement was tested at the same wavelength by comparing ZnGeP2 with HgGa2S4 which led to the result d36(ZnGeP2)~3d36(HgGa2S4), in very good agreement with previous estimations.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.