We show that massless solutions to the Einstein-Vlasov system in a Bianchi I space-time with small anisotropy, i.e. small shear and small trace-free part of the spatial energy momentum tensor, tend to a radiation fluid in an Einstein-de Sitter space-time with the anisotropy $Sigma^a_bSigma^b_a$ and $tilde{w}^i_j tilde{w}^j_i$ decaying as $O(t^{-frac12})$.
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