In this paper we use new CCD observations to examine the accuracy of the magnitude system we have adopted to define the galaxy sample being used to conduct the Southern Sky Redshift Survey Extension hereafter referred to as (SSRS2). The magnitude scale of the SSRS2 is based on the instrumental magnitudes of the nonstellar objects listed in the ST ScI Guide Star Catalog, converted to the B(0) (Zwicky) system. This was done using a linear relation derived from the comparison of objects in common with the photometric catalogue of Lauberts & Valentijn [The Surface photometry Cataloque, of the ESO-Uppsala Galaxies (European Southern Observatory, Munchen) (1989)]. Using the results of new independent CCD observations we find: (1) the magnitudes derived by Lauberts & Valentijn are in excellent agreement with the CCD measurements, with isophotal magnitudes showing a dispersion of ~0.13 mag, although an offset 0.15 mag in zero point may be present; (2) the magnitudes m_SSRS2_ derived from the ST ScI instrumental magnitudes correspond well to a blue magnitude measured within B = 26 mag/arcsec^2^ isophote. These magnitudes have an intrinsic dispersion of about 0.34 mag, comparable to that estimated for the Zwicky system, and a relatively small plate-to-plate variation of the zero point. These results indicate that the ST ScI objects may provide an invaluable source for the construction of a uniform magnitude-limited sample of galaxies in the southern hemisphere.