  We present a measurement of the muon charge asymmetry from W boson decays
using 0.3 fb^{-1} of data collected at \sqrt{s}=1.96 GeV between 2002 and 2004
with the D0 detector at the Fermilab Tevatron ppbar Collider. We compare our
findings with expectations from next-to-leading-order calculations performed
using the CTEQ6.1M and MRST04 NLO parton distribution functions. Our findings
can be used to constrain future parton distribution function fits.
