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We present VI photometry for the brightest stars in the Virgo spiral galaxy M100 (= NGC 4321) obtained with the WFPC2 on the Hubble Space Telescope. A total 25 F555W images were obtained over 12 epochs spanning 57 days, while 9 F814W images were obtained over 4 epochs during the same interval. The photometric reduction routines DAOPHOT and DoPHOT were independently used to obtain instrumental magnitudes for the brightest stars in the galaxy. A comparison of the two methods with each other and with aperture photometry shows that both routines give reliable results for the undersampled WFPC2 data. We have examined in detail the transformation from the WFPC2 instrumental photometric system to the standard VI system and show that the calibration provided by the instrument development team is in good agreement with an independent ground-based calibration. The color-magnitude diagram and luminosity function for M100 show that the luminous stellar population of the galaxy is similar to that of other, more nearby spiral galaxies. These data were obtained as part of the Hubble Space Telescope Key Project on the extragalactic distance scale, the goal of which is to determine the Hubble constant to within an accuracy of 10\%.
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