We measure the branching fraction of the top quark to
longitudinally and right-handed polarized W bosons, F_0 and F_+, using
approximately 200 pb-1 of p anti-p collisions collected by the CDF II
detector.  We analyze two quantities sensitive to the W helicity: the
invariant mass of the charged lepton and the bottom-quark jet in the decay
t --> Wb --> l nu b (where l = e or mu), and the transverse momentum of the
charged lepton.  We find F_0 = 0.74+0.22_-0.34, and F_+ < 0.27 at the 95%
confidence level.  These measurements are in agreement with the standard
model predictions.