The extragalactic distance scale key project. XVI. Cepheid variables in aninner field of M101

We report on the identification of 255 candidate variable stars in a field located some 1.'7 from the center of the late-type spiral galaxy M101 = NGC 5457, based on observations made with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. Photometric measurements in the F555 W and F814W filters-analyzed independently with the DAOPHOT/ALLFRAME and Do PHOT software suites-have been transformed to the Johnson V and Kron-Cousin s I standard magnitude systems. Periods and intensity-averaged mean magnitu des for 61 carefully selected candidate Cepheid variables with periods in t he range 10-48 days indicate a reddening-corrected mean distance modulus (m - M)(o) = 29.05 +/- 0.14 (if the true modulus of the Large Magellanic Clou d is 18.50 +/- 0.10, and if there is no dependence of the period-luminosity relation on metal abundance); results consistent with this are obtained wh ether or not the sample is expanded to include a larger fraction of the can didates. Applying a metallicity-dependent correction of +0.16 +/- 0.10 mag would increase this estimate to (m - M)(o) = 29.21 +/- 0.17 mag.