A first analysis of the full data set collected by CHORUS in the years 1994
-1997 is presented. The signal of nu (tau) appearance in an almost pure nu
(mu) beam has been searched looking for tau leptons produced in nu (tau) ch
arged current interactions and their subsequent decay into both the leptoni
c and hadronic decay channels. No tau candidate has been found. A nu (mu) -
-> nu (tau) mixing is excluded down to sin(2)2 theta (mu tau) = 6.8 x 10(-4
) for large Deltam(2) (90% C.L.).