Femtosecond pump-probe spectroscopy was used to directly monitor, in the time domain, energy transfer from the blue-emitting poly(9,9-dioctylfluorene) conjugated polymer to the red-emitting tetraphenylporphyrin molecule. This system is under investigation for use as saturated red light-emitting component in an organic full-color display. The differential transmission spectrum of the system evolves, within a few picoseconds after photoexcitation, from that of the host polymer to that of the guest molecule. Experiments performed at different acceptor concentrations provide clear evidence for a Forster energy transfer mechanism and allow a determination of the interaction radius. A value of 4.2 +/- 0.2 nm was found.