Fritz Gesztesy, Karl Unterkofler, and Rudi Weikard An Explicit Characterization of Calogero--Moser Systems (173K, LaTeX) ABSTRACT. Combining theorems of Halphen, Floquet, and Picard and a Frobenius type analysis, we characterize rational, meromorphic simply periodic, and elliptic KdV potentials. In particular, we explicitly describe the proper extension of the Calogero--Moser locus associated with these three classes of algebro-geometric solutions of the KdV hierarchy with special emphasis on the case of multiple collisions between the poles of solutions.