Photoelectric UBVRI observations are given for 336 stars brighter than 11.5mag from the Hubble Space Telescope Guide Star Photometric Catalogue (GSPC). Apart from a Declination-dependent effect in the V magnitudes there are no systematic differences between GSPC (Cat. <II/143>) and SAAO (Menzies et al., 1989SAAOC..13....1M) data which are significantly greater than differences between the Landolt and E-region standard systems. The Declination effect might be due to the occurrence of unusually high atmospheric extinction coefficients at CTIO following the eruption of the Mexican volcano, El Chichon.