This proposal requests an x-ray diffractometer and associated data system for crystal structure solution and refinement to serve the needs of a group of researchers in the Yale Chemistry Department involved in NIH-sponsored research in chemical synthesis. This system is to be optimized for determination of the structures of small organic molecules, the intermediates and synthetic targets in the research programs of the investigators. These projects involve synthesis and characterization of a wide range of molecules of biological importance, including macrolides, vitamins, tetramic acids, lactam antibiotics, carbohydrates, steroids, alkoloids and other natural products. The proposed instrument will be placed in the Yale Chemical Instrumentation Center, where a staff crystallographer will maintain and operate the diffractometer and provide crystal structure determination services to the investigators.