Kidney bean purple acid phosphatase (KBPAP) is an Fe(III)-Zn(II) metalloenzyme resembling the mammalian Fe(III)-Fe(II) purple acid phosphatases. The structure of the homodimeric 111-kilodalton KBPAP was determined at a resolution of 2.9 angstroms. The enzyme contains two domains in each subunit. The active site is located in the carboxyl-terminal domain at the carboxy end of two sandwiched beta alpha beta alpha beta motifs. The two metal ions are 3.1 angstroms apart and bridged monodentately by Asp164. The iron is further coordinated by Tyr167, His325, and Asp135, and the zinc by His286, His323, and Asn201. The active-site structure is consistent with previous proposals regarding the mechanism of phosphate ester hydrolysis involving nucleophilic attack on the phosphate group by an Fe(III)-coordinated hydroxide ion.