  The decay branching fractions of the three narrow Upsilon resonances to mu+
mu- have been measured by analyzing about 4.3 1/fb e+e- data collected with the
CLEO III detector. The branching fraction B(Y(1S) -> mu+ mu-) = (2.49 +- 0.02
+- 0.07)% is consistent with the current world average but B(Y(2S) -> mu+ mu-)
= (2.03 +- 0.03 +- 0.08)% and B(Y(3S) -> mu+ mu-) = (2.39 +- 0.07 +- 0.10)% are
significantly larger than prior results. These new muonic branching fractions
imply a narrower total decay width for the Y(2S) and Y(3S) resonances and lower
other branching fractions that rely on these decays in their determination.
