We present a catalog of 203 clusters of galaxies serendipitously detected in 647 ROSAT PSPC high Galactic latitude pointings covering 158deg^2^. This is one of the largest X-ray-selected cluster samples, comparable in size only to the ROSAT All-Sky Survey sample of nearby clusters (Ebeling et al., 1997ApJ...479L.101E). We detect clusters in the inner 175 of the ROSAT PSPC field of view using the spatial extent of their X-ray emission. Fluxes of detected clusters range from 1.6x10^-14^ to 8x10^-12^ergs/s/cm^2^ in the 0.52keV energy band. X-ray luminosities range from 10^42^ergs/s, corresponding to very poor groups, to ~5x10^44^ergs/s, corresponding to rich clusters. The cluster redshifts range from z=0.015 to z>0.5. The catalog lists X-ray fluxes, core radii, and spectroscopic redshifts for 73 clusters and photometric redshifts for the remainder.
