In plants, the hormone indole-3-acetic acid (IAA) can initiate the developmental program for lateral root formation. We have isolated mutants that have permitted the dissection of this program into initiation and maturation of lateral roots. The alf1-1 mutation causes hyperproliferation of lateral roots, alf4-1 prevents initiation of lateral roots, and alf3-1 is defective in the maturation of lateral roots. The alf3-1 mutant can be rescued by IAA, whereas the alf4-1 mutant is not rescued. Our data suggest a model in which IAA is required for at least two steps in lateral root development: (1) to initiate cell division in the pericycle, and (2) to promote cell division and maintain cell viability in the developing lateral root.