The space D λ;µ , where λ = (λ 1 , . . . , λ m ), of m-ary differential operators acting on weighted densities is a (m + 1)-parameter family of modules over the Lie algebra of vector fields. For almost all the parameters, we construct a canonical isomorphism between the space D λ;µ and the corresponding space of symbols as sl(2)-modules. This yields to the notion of the sl(2)-equivariant symbol calculus for m-ary differential operators. We show, however, that these two modules cannot be isomorphic as sl(2)-modules for some particular values of the parameters. Furthermore, we use the symbol map to show that all modules D 2 λ;µ (i.e., the space of second-order operators) are isomorphic to each other, except for few modules called singular.