Excited-state dynamics and exciton delocalization in the isolated LH2 antenna complex (B800-850) of the photosynthetic bacterium Rhodopseudomonas acidophila were studied at low temperature by means of femtosecond transient absorption spectroscopy. By comparing the contribution by stimulated emission to the absorbance difference signal of B850 with the bleaching of the primary donor P of reactions centers of Rhodobacter sphaeroides R26, we found that at 5 K the thermalized B850 transition has a 1.85 times larger oscillator strength than P. This corresponds to an oscillator strength equivalent to 3.4 reaction center pigments. With the in vivo extinction coefficients for the reaction center and LH2 from the literature, we arrive at an oscillator strength equivalent to 2.3 antenna pigments. Comparison of the stimulated emission band of B850 with the optical signal of B800 yields an oscillator strength equivalent with that of 2.7 B800 pigments. Our results strongly support a model in which the exciton states are partly localized as a result of static disorder. Energy-selective steady-state fluorescence measurements indeed indicate inhomogeneity of the B850 band. Energy transfer from B800 to B850 takes place with a time constant of 1.8 ps at 10 K.