We present a measurement of the fractions F_0 and F_+ of
longitudinally polarized and right-handed W bosons in top quark decays using
data collected with the CDF II detector.  The data set used in the analysis
corresponds to an integrated luminosity of approximately 318 pb-1.  We
select tt-bar candidate events with one lepton, at least four jets, and
missing transverse energy.  Our helicity measurement uses the decay angle
theta*, which is defined as the angle between the momentum of the charged
lepton in the W boson frame and the W momentum in the top quark rest frame.
The cos theta* distribution in the data is determined by full kinematic
reconstruction of the tt-bar candidates.  We find F_0 =
0.85+0.15_-0.22(stat) +/- 0.06(syst) and F_+ = 0.05+0.11_-0.05(stat) +/-
0.03(syst), which is consistent with the standard model prediction.  We set
an upper limit on the fraction of right-handed W bosons of F_+ < 0.26 at the
95% confidence level.