In the Novikov-Krichever formula for a fourth-order operator L 4 , occurring in a commuting pair of pank two and genus one, there is an arithmetical error; the operator L 4 has the form(one of the terms d/dx a has been omitted), whereFor all the quotations and formulas see the paper by Grinevich in this issue.Theorem 1. The operator L 4 is formally symmetric if and only if a = 0, i.e., λ 1 = λ 2 (the constant γ 0 = 0 modulo the semiperiods of the function ℘).In this case the formulas for the coefficients are strongly simplified:(2)Assume that the following conditions hold : 1) the Riemann surface Γ is real (i.e., g 1 , g 2 are real ); 2) the function λ(x) is real ; 3) at the points where λ = 0 we have λ = 0, λ 2 = P 3 (λ). Then, the operator L 4 , defined by (1), (2), has nonsingular real periodic coefficients and is a semibounded self-adjoint operator in L 2 (R).For any linear ordinary differential operator L (with respect to x) with periodic coefficients there is defined a "monodromy matrix"T (λ), i.e., a shift operator on the period of solutions of the equation Lψ = λψ, written in a certain basis. The order of the matrixT (λ) is equal to the order k of the operator L. The eigenvectors ψ q (x, λ) of the operatorT (λ) are called the Bloch eigenfunctions (or the Floquet functions). The function ψ q (x, λ) is meromorphic in λ on the Riemann surfaceΓ over the λ-plane of k sheets, whose points are the pairs Q q = (λ, q), q = 1, . . . , k.Theorem 3. Assume that the operator L of order k = nl occurs in the commuting pair [A, L] = 0 of rank l with Riemann surface Γ of finite genus {P (A, L) = 0}. Then, the monodromy matrixT (λ), λ ∈ C, written in the basis of the BakerAkhiezer eigenfunctions ϕ α (x, P i ), α = 1, . . . , l, defines a matrixT * (P) of order