The conversion efficiency of the first Stokes radiation generated by backward stimulated Raman scattering is measured in CH(4) and H(2) that are pumped by a third harmonic of a Nd:YAG laser in the 12-55-mJ energy range and in the 0.86-6.00-MPa pressure range. Our experimental results show that CH(4) is a better Raman medium than H(2) at moderate pressures (a few megapascals) to efficiently generate backward Raman lines.