The crystal structure of an alpha-spectrin Src-homology 3 (SH3) domain mutant has been refined at 1.12 A resolution. This X-ray structure is at the highest resolution achieved so far for an SH3 domain. The structure allows the identification of a complete set of specific interactions and is useful for the elucidation of relations between structure and pH-dependent thermodynamic stability in a series of SH3 domain mutants.