The light-induced electron transport in purple bacterium Rhodobacter sphaeroides was studied in vivo by means of kinetic difference absorption spectroscopy and kinetics of bacteriochlorophyll fluorescence yield. Measurements of redox state of the oxidised primary donor and cytochrome c and the membrane potential revealed a complex pattern of changes of the electron flow. Effects of the membrane potential on the fluorescence yield were also analysed, and a model for the fluorescence induction curve is presented. The data indicate substantial positive effect of the membrane potential on the fluorescence emission in vivo. Moreover, light-induced changes in light scattering were observed, which suggests occurrence of structural changes on the level of the photosynthetic membrane.