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We have obtained Hubble Space Telescope (HST) images of the Virgo galaxy M100 to identify and measure Cepheid variables, and employ the Cepheid Period-Luminosity relation to determine a primary distance to this galaxy. The observations are part of the HST Key Project on the Extragalactic Distance Scale, which aims to provide an absolute calibration for a number of secondary distance indicators (including, for example, the Tully-Fisher relation and the Expanding Photosphere Method) by measuring Cepheid distances to approximately 20 galaxies within a redshift of $\sim$ 1500 km s$^{-1}$.
A total of 25 $V$ band and 9 $I$ band exposures of M100 were obtained at 12 $V$ and 4 $I$ epochs, using the Wide Field and Planetary Camera 2 in the first half of 1994, spanning a time interval of approximately 2 months. ALLFRAME (Stetson 1993) and a variant of DoPHOT (Schechter et al 1993) were used independently to perform the photometric reduction of the frames. Approximately 60 Cepheids have been found so far, from which a preliminary distance to M100 of 17.1 $\pm$ 1.8 Mpc is derived (Freedman et al, 1994, Ferrarese et al. 1995). This distance is in good agreement with the distance calculated using the Tully-Fisher relation (Pierce and Tully 1988), and with the distance inferred from SN1979C by applying the Expanding Photosphere Method (Schmidt, Kirshner, Eastman 1992).
