The quantum yield of the retinal photoisomerization, the primary step in the bacteriorhodopsin (bR) photosynthesis, is determined from the picosecond bleach recovery of the absorption at 560 nm in the pH range 6.8-11.3. From this and the dependence of the observed retinal excited state decay on pH, the effects of pH on both the photoisomerization and internal conversion rates are also determined. The results show that while the quantum yield of photoisomerization is independent of pH, each of the rate constants decreased by more than a factor of 2 at elevated pH. This suggests that the perturbation resulting from the pH changes affects both rate constants similarly. This observation is discussed in terms of the theoretically proposed potential energy surfaces of the ground and excited states of retinal in bR.