Circular dichroism spectra of Fenna-Matthews-Olson trimers (bacteriochlorophyll a-protein antenna complexes from green photosynthetic bacteria) are unusually sensitive to diagonal energy disorder because the resonance interactions between pigments belonging to different protein subunits are weak (less than or equal to 20 cm(-1)). Substantial differences occur between CD spectra in exciton simulations performed with and without realistic inhomogeneous distribution functions for the input pigment diagonal energies. Future simulations of FMO optical properties such as CD and pump-probe anisotropies should consider diagonal disorder in order to accurately describe this antenna’s electronic structure.