The theory for the effects of crowding on the behavior of reversibly self-assembling solutes is extended to mixtures containing nonassembling solutes. The theory predicts that excluded volume will cause dramatic demixing into domains of long, tightly packed, highly aligned fibers coexisting with an isotropic solution of unaggregated species. It suggests that the bundling of fibers in cells is entropically driven and that accessory binding proteins in the cytoplasm serve to modulate the process rather than create it.