This is an all-sky catalog of 4073 rich clusters of galaxies, each having at least 30 members within the magnitude range m_3_ to m_3_ + 2 (m_3_ is the magnitude of the third brightest cluster member) and each with a nominal redshift less than 0.2. The southern data have been collected from a survey of UK 1.2 m Schmidt telescope IIIa-J plates and films and have been reduced to the systems defined by the northern data previously published by G.O. Abell. A revised northern catalog, including Bautz-Morgan types and redshifts where known, is also included. Nomenclature Notes: The recommended acronym used to designate the clusters from this list (the "Abell" clusters) is: ACO (numbers 1 through 4076, 1-2712 being Northern, 2713-4076 Southern) ACO S (numbers 1 through 1174), supplementary Southern clusters. In the literature, the following acronyms have also been found: A , Abell, ABCG, AC, S (the last for supplementary Southern clusters)