  Recent research suggests that the charge carriers in the paramagnetic state
of the magnetoresistive manganites are small polarons. Here we report studies
of the oxygen-isotope effects on the intrinsic resistivity and thermoelectric
power in several ferromagnetic manganites. The precise measurements of these
isotope effects allow us to make a quantitative data analysis. Our results do
not support a simple small-polaron model, but rather provide compelling
evidence for the presence of small immobile bipolarons, i.e., pairs of small
polarons. Since the bipolarons in the manganites are immobile, the present
result alone appears not to give a positive support to the bipolaronic
superconductivity theory for the copper-based perovskites.
