A self-assembled gel, prepared in an organic solvent from two structurally related gelators, consists of fibers made exclusively of one compound and microcrystallites made exclusively of the other according to microscopic, calorimetric, and rheological information. The high selectivity occurs despite the two gelators having identical hydrogen-bonding units critical to the self-assembly process. Seemingly innocuous sidechain effects apparently suffice for a growing fiber to accept molecule 1, and to reject molecule 2, with a high degree of fidelity.