The effect of polymer polydispersity on micellization in an aqueous solution of triblock copolymers containing ethylene oxide (EO) and propylene oxide (PO) was modeled. A mean-field lattice theory for multicomponent mixtures of copolymers with internal degrees of freedom was applied. The polydisperse polymer was represented as consisting of several components, and a Schulz-Zimm distribution was assumed. The critical micellar concentration (cmc) and the aggregation number number were examined for a number of PEO-PPO-PEO polymers at different temperatures and with different polydispersity ratios. Segment density profiles indicated the polymer micelles to consist of a hydrophobic core composed mainly of PO and an outer layer composed of a mixture of EO and water. The polydispersity led to a reduction in the cmc by several orders of magnitude, which brought experimental and calculated data close to each other. The precise value of the cmc was found to depend strongly on the criterion of the cmc. The polydispersity also led to an increase in micellar size, a greater separation of the EO and PO segments, and a marked decrease in micellar size with increasing total polymer concentration. The fractions of the different polymer components of the micelles depended strongly on the polymer concentration. Close to the cmc, the micelles were predominantly formed by the longest components, which had the lowest cmc. At high polymer concentrations, the intermicellar solution became strongly depleted of the long polymer components. The strong temperature dependence of the cmc and of the aggregation number was not changed essentially by polydispersity.