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We present a catalog of 200 clusters of galaxies serendipitously detected in647 ROSAT PSPC pointings covering 158 square degrees. This is one of thelargest X-ray selected cluster samples, comparable in size only to the ROSATAll-Sky Survey sample of nearby clusters (Ebeling et al. 1997). We detectclusters in the inner 17.5 arcmin of the ROSAT PSPC field of view using thespatial extent of their X-ray emission. Cluster X-ray luminosities range from10^42 erg/s to ~5x10^44 erg/s, i.e. from poor groups to rich clusters. Thecluster redshifts range from z=0.015 to z>0.5. The catalog lists X-ray fluxes,core-radii, spectroscopic redshifts for 73 clusters and photometric redshiftsfor the remainder. We have optically confirmed 200 of 223 X-ray sources asclusters of galaxies. Of the remaining 23 sources, 18 are likely falsedetections arising from blends of unresolved point X-ray sources, and for 5 wehave not obtained deep CCD images. Above a flux of 2e-13 erg/s/cm**2, 98\% ofextended X-ray sources are optically confirmed clusters. The log N-log Srelation derived from our catalog shows excellent agreement with previousresults: EMSS and ROSAT All-Sky survey BCS in teh bright end and WARPS atintermediate fluxes. Our cluster counts appear to be systematically higher thanthose from a 50 deg^2 survey of Rosati et al. (1998). In particular, at a fluxof 2e-13 erg/s/cm**2, we find a surface density of clusters of 0.57+/-0.07 persquare degree, which is a factor of 1.3 more than found by Rosati et al. Thisdifference is marginally significant at the ~2 sigma level. The large area ofour survey makes it possible to study the evolution of the X-ray luminosityfunction in the high luminosity range inaccessible with other, smaller areaROSAT surveys.Comment: To appear in Aug 1, 1998 issue of ApJ (vol 502

A Catalog of 203 Galaxy Clusters Serendipitously Detected in theROSATPSPC Pointed Observations