Abstract. We prove that the 2-primary π 61 is zero. As a consequence, the Kervaire invariant element θ 5 is contained in the strictly defined 4-fold Toda bracket 2, θ 4 , θ 4 , 2 .Our result has a geometric corollary: the 61-sphere has a unique smooth structure and it is the last odd dimensional case -the only ones are S 1 , S 3 , S 5 and S 61 .Our proof is a computation of homotopy groups of spheres. A major part of this paper is to prove an Adams differential d 3 (D 3 ) = B 3 . We prove this differential by introducing a new technique based on the algebraic and geometric Kahn-Priddy theorems. The success of this technique suggests a theoretical way to prove Adams differentials in the sphere spectrum inductively by use of differentials in truncated projective spectra.