The Euler–Lagrange equations in classical field theory are not covariant with respect to a transformation of independent variables. A modified covariant version of these equations is presented. The modification consists of replacing the partial derivative ∂/∂qiby the Hermitian quantum mechanical momentum operatorpiconjugate to the coordinateqi. This momentum operator is given bypi=−ih/[∂/∂qi+(1/2) ∂/∂qi(lnJ)], whereJis the Jacobian of the transformation.