The spectroscopic, photochemical, and energetic properties of the primary event of light-adapted bacteriorhodopsin (bR) are investigated with pulsed laser cryogenic photocalorimetry, photostationary-state spectral analysis, INDO-PSDCI molecular orbital theory, and semiempirical molecular dynamics theory. The principal goal is to explore the photophysical origins of the controversy concerning the primary quantum yield. ... Experimental and theoretical evidence is presented which suggests that two distinct forms of light-adapted bacteriorhodopsin may exist. We propose that these two forms have characteristic photocycles with significantly different primary quantum yields.