When dissolved in N,N-dimethylformamide and then dialyzed against phosphate-buffered saline, A-B-A block copolymers composed of poly[M-(2-hydroxyethyl)-LglutamineJ-block-poly(v-benzyl-L-glutamate)-block-poly[N5-(2-hydroxyethyl)-L-glutamineJ form particles. The particles are cage-like structures with average diameters of 300 am (average polydispersity, 0.3-0.5). They are stable in aqueous solution at 4(C for up to 3 weeks, at which time flocculation becomes apparent. Negative staining and freeze-fracture electron microscopy suggest that cage-like particles are formed by selective association of segregated micelle populations. A model of particle formation is presented in which B blocks form micelles in dimethylformamide. On dialysis against an aqueous solution, the extended A blocks then associate intermolecularly to form rod-shaped micelles, which connect the B block micelles. The result is a meshed cage-like particle. The implications of these observations on the aggregation behavior of polymeric surfactants in dilute solution are discussed.Amphiphilic block copolymers, containing hydrophobic and hydrophilic components, exhibit surfactant behavior (1, 2) and may form micelles. Micelles are also formed by block copolymers in organic solvents when one of the blocks is insoluble (3). In this case, the insoluble blocks form the micelle core and are surrounded by the soluble blocks. Elongated or rod-shaped micelles rather than globular micelles may be formed by block copolymers in dilute solution (4, 5). These elongated structures may be considered to be extended tubes having insoluble cores surrounded by soluble shells. Such micelles have been described for systems such as polystyrene-block-poly(methylmethacrylate) in toluene/furfuryl alcohol, polystyreneblock-polybutadiene-block-polystyrene copolymer in isopropyl acetate (6), polystyrene-block-polybutadiene in homopolystyrene (7), and polystyrene-block-polyisoprene copolymer in N,N-dimethylacetamide (8). Elongated micelles have been documented by electron microscopy (9).In existing studies of amphiphilic polypeptide block copolymers, little attention has been given to micelles formed in dilute solution by these surfactants (10, 11). Only recently has this problem been considered for A-B, A-B-A, and A-B-C block copolymers incorporating flexible and rigid, rod-like blocks in solvents good for the flexible blocks but that are nonsolvents for the rod-like blocks (12).We have observed that a poly(amino acid)-based A-B-A block copolymer will form stable cage-like particles in dilute aqueous solution when prepared through the solvent N,Ndimethylformamide (DMF). A model is proposed that explains the formation of the cage particles in terms of ordered and selective interactions of segregated populations of micelles. This model, based on the solution behavior of amphiphiles (13), is consistent with models recently proposed for other amphiphilic copolymers (14, 15)