[en] A first analysis of the full data set collected by CHORUS in the years 1994-1997 is presented. The signal of ν_τ appearance in an almost pure ν_μ beam has been searched looking for τ leptons produced in ν_τ charged current interactions and their subsequent decay into both the leptonic and hadronic decay channels. No τ candidate has been found. A ν_μ → ν_τ mixing is excluded down to sin²2θ_μτ = 6.8 x 10^-4 for large Δm² (90% C.L.)