  We search for evidence of a light scalar boson in the radiative decays of the
Upsilon(2S) and Upsilon(3S) resonances: Upsilon(2S,3S)-> gamma A0, A0 -> mu^+
mu^-. Such a particle appears in extensions of the Standard Model, where a
light CP-odd Higgs boson naturally couples strongly to b-quarks. We find no
evidence for such processes in the mass range 0.212<= m(A0)<= 9.3 GeV in the
samples of 99*10^6 Upsilon(2S) and 122*10^6 Upsilon(3S) decays collected by the
BABAR detector at the PEP-II B-factory and set stringent upper limits on the
effective coupling of the b quark to the A0. We also limit the dimuon branching
fraction of the eta_b meson: BR(eta_b->mu^+mu^-)<0.9% at 90% confidence level.
