We have performed a kinetic and thermodynamic analysis of 3D(pol) derivatives containing substitutions in the ribose-binding pocket with ATP analogues containing correct and incorrect sugar configurations. We find that Asp-238, a residue in structural motif A that is conserved in all RNA-dependent RNA polymerases, is a key determinant of polymerase fidelity. Alterations in the position of the Asp-238 side chain destabilize the catalytically competent 3D(pol)-primer/template-NTP complex and reduce the efficiency of phosphoryl transfer. The reduction in phosphoryl transfer may be a reflection of increased mobility of other residues in motif A that are required for stabilizing the triphosphate moiety of the nucleotide substrate in the active conformation. We present a structural model to explain how Asp-238 functions to select nucleotides with a correct sugar configuration and a correct base. We propose that this mechanism is employed by all RNA-dependent RNA polymerases. We discuss the possibility that all nucleic acid polymerases with the canonical "palm"-based active site employ a similar mechanism to maximize fidelity.