This "catalog of principal galaxies" constitutes the basis of the "Third Reference Catalogue of Bright Galaxies" (RC3). It lists equatorial coordinates for the equinoxes 1950 and 2000 and cross identifications for 73197 galaxies. Of the 73197 galaxies, 40932 have coordinates with standard deviations of less than ten arcsec. Listed are 131,601 names from the 38 most common sources. These data are given when available: morphological descriptions, apparent major and minor axes, apparent magnitudes, radial velocities, and position angles. Related data: VII/137B : Third Reference Catalogue of bright galaxies (RC3)