[en] We use the transverse momentum spectrum of leptons in the decay chain t→bW with W→lν to measure the helicity of the W bosons in the top quark rest frame. Our measurement uses a tt sample isolated in 106±4 pb^-1 of data collected in pp collisions at √(s)=1.8 TeV with the CDF detector at the Fermilab Tevatron. Assuming a standard V-A weak decay, we find that the fraction of W 's with zero helicity in the top rest frame is F₀=0.91±0.37(stat )±0.13(syst) , consistent with the standard model prediction of F₀=0.70 for a top mass of 175 GeV/c² . (c) 2000 The American Physical Society