The USNO CCD Astrograph (UCA) started an astrometric survey in 1998 February at Cerro Tololo, Chile. This Ðrst, preliminary catalog (UCAC1) includes data taken up to 1999 November with about 80% of the Southern Hemisphere covered. Observing continues, and full sky coverage is expected by mid-2003 after moving the instrument to a Northern Hemisphere site in early 2001. The survey is performed in a single bandpass (579-642 nm), a twofold overlap pattern of Ðelds, and with a long and a short exposure on each field. Stars in the magnitude range 10-14 have positional precisions of <=20 mas. At the limiting magnitude of RB 16 mag, the positional precision is 70 mas. The UCAC aims at a density (stars per square degree) larger than that of the Guide Star Catalog (GSC) with a positional accuracy similar to Tycho. The UCAC program is a major step toward a high-precision densiÐcation of the optical reference frame in the post Hipparcos era, and the first stage, the UCAC1 contains over 27million stars. Preliminary proper motions are included, which were derived from Tycho-2, Hipparcos, and ground-based transit circle and photographic surveys for the bright stars (V <=12.5 mag) and the USNO A2.0 for the fainter stars. The accuracy of the proper motions varies widely, from 1 to over 15 mas yr~1.