Electronic Journal of Differential Equations, Vol. 2000(2000), No. 19, pp. 1-12. Title: On commuting differential operators Author: R. Weikard (Univ. of Alabama at Birmingham, Alabama, USA) Abstract: The theory of commuting linear differential expressions has received a lot of attention since Lax presented his description of the KdV hierarchy by Lax pairs $(P,L)$. Gesztesy and the present author have established a relationship of this circle of ideas with the property that all solutions of the differential equations $Ly=zy$, $z\in {\Bbb C}$, are meromorphic. In this paper this relationship is explored further by establishing its existence for Gelfand-Dikii systems with rational and simply periodic coefficients. Submitted February 22, 2000. Published March 9, 2000. Math Subject Classifications: 34M05, 37K10, 37K20. Key Words: Meromorphic solutions; Commuting differential expressions; Lax pairs; KdV; Gelfand-Dikii systems.