Abstract. We characterize in terms of Darboux transformations the spaces in the Segal-Wilson rational Grassmannian, which lead to commutative rings of differential operators having coefficients which are rational functions of e x . The resulting subgrassmannian is parametrized in terms of trigonometric Calogero-Moser matrices.2000 Mathematics Subject Classification. 35Q53, 37K101. Introduction. The rational Segal-Wilson Grassmannian Gr rat parametrizes the soliton solutions of the Kadomtsev-Petviashvili (KP) equation. In [10], Wilson embarked upon a study of a subgrassmannian Gr ad ⊂ Gr rat that he called the adelic Grassmannian, which parametrizes the solutions of the KP equation, rational in x and vanishing as x → ∞. The adelic Grassmannian has (and is indeed characterized by) a remarkable bispectral involution V → b(V ), V ∈ Gr ad , which exchanges the role of the 'space' and the 'spectral' variables in the corresponding stationary wave functions ψ V (x, z), that is