The crystal structure of dihydrofolate reductase (EC 1.5.1.3) from Escherichia coli has been solved as the binary complex with NADP+ (the holoenzyme) and as the ternary complex with NADP+ and folate. The Bragg law resolutions of the structures are 2.4 and 2.5 A, respectively. The new crystal forms are nonisomorphous with each other and with the methotrexate binary complex reported earlier [Bolin, J. T., Filman, D. J., Matthews, D. A., Hamlin, R. C., & Kraut, J. (1982) J. Biol. Chem. 257, 13650-13662]. In general, NADP+ and folate binding conform to predictions, but the nicotinamide moiety of NADP+ is disordered in the holoenzyme and ordered in the ternary complex. A mobile loop (residues 16-20) involved in binding the nicotinamide is also disordered in the holoenzyme. We report a detailed analysis of the binding interactions for both ligands, paying special attention to several apparently strained interactions that may favor the transition state for hydride transfer. Hypothetical models are presented for the binding of 7,8-dihydrofolate in the Michaelis complex and for the transition-state complex.