The Ly49 family of natural killer (NK) receptors regulates NK cell function by sensing major histocompatibility complex (MHC) class I. Ly49 receptors show complex patterns of MHC class I cross-reactivity and, in certain cases, peptide selectivity. To investigate whether specificity differences result from topological differences in MHC class I engagement, we determined the structure of the peptide-selective receptor Ly49C in complex with H-2K(b). The Ly49C homodimer binds two MHC class I molecules in symmetrical way, a mode distinct from that of Ly49A, which binds MHC class I asymmetrically. Ly49C does not directly contact the MHC-bound peptide. In addition, MHC crosslinking by Ly49C was demonstrated in solution. We propose a dynamic model for Ly49-MHC class I interactions involving conformational changes in the receptor, whereby variations in Ly49 dimerization mediate different MHC-binding modes.