We have simulated the excited stare dynamics of the FMO (Fenna-Matthews-Olson) bacteriochlorophyll a-protein complexes of the green sulfur bacteria Chlorobium (C.) tepidum and Prosthecochloris (P.) aestuarii at cryogenic temperature in terms of an exciton model. The simulation is based on the electronic structure, as described in previous publications (Louwe, R. J. W.; Vrieze, J.; Hoff, A. J.; Aartsma, T. J.; J. Phys. Chem. 1997, 101, 11280. Vulto, S. I. E.; de Baat, M. A.; Louwe, R. J. W.; Permentier, Ii. P.; Neef, T.; Miller, M.; van Amerongen, Ii.; Aartsma, T. J. J. Phys. Chem. 1998, 102, 9577). Relaxation between exciton states is described by linear electron phonon coupling as a perturbation term in the Hamiltonian of the system. The simulation was compared with experimental data obtained by pump-probe measurements with various wavelengths of excitation. For C. tepidum, a quite good agreement was obtained between the calculated and measured dynamics. For P. aestuarii, the simulations are less satisfactory but they can be improved by including static disorder. We conclude that not only the steady-state optical spectra but also the excited-state dynamics in the FMO complex at low temperature can be described with a simple exciton model.