Let U = C 2 , Γ = Z 2 , and let C[x ±1 1 , x ±1 2 ] be the ring of Laurent polynomials. The Witt algebra L is the Lie algebra of derivations overgives a class of L -modules, often called the Larsson-modules of L . In this paper, we study the derivations from the Witt algebra L to its Larsson-modules W , and we determine the first cohomology group H 1 (L , W ).