Oscillatory components of the absorption changes in reaction centers from Rhodobacter sphaeroides are observed by femtosecond spectroscopy in the spectral region between 920 and 1100 nm covering the ranges of excited-state absorption and stimulated emission of the photoexcited primary donor and the long wavelength absorption band of the bacteriochlorophyll anion. These oscillations with a dominant frequency of 135 cm(-1) are damped on the 0.4-0.8 ps time scale. The observations fit well into a model with vibrational wave packet motion on the excited-state potential surface modulating stimulated emission. Amplitudes and phases of the oscillation do not show any peculiarity within the absorption band of the bacteriochlorophyll anion, indicating that the interaction of this wave packet with electron transfer to the accessory bacteriochlorophyll is not required to explain the experimental data.