The transition from vegetative to reproductive growth represents a major change in angiosperm development. During the floral transition, shoot meristem activity progresses through the overlapping developmental phases: vegetative→inflorescence→floral. The transition to flowering is regulated by graft-transmissible substances, and the arrival of these substances at the shoot apex is correlated with the establishment of the inflorescence meristem. To identify the genes involved in regulating the floral transition, we have screened for mutants of Arabidopsis thaliana that display altered timing of flowering. We expect that such heterochronic mutants are defective either in the production or transport of floral signals, or in the response of the shoot meristem to these signals. We have isolated four early-flowering mutants: terminal flower 1 (tfl1) and early-flowering (elf) 1, 2 and 3. Three phenotypic classes are represented by these mutations: (I) early-flowering, photoperiod-sensitive, and displaying abnormal inflorescence development (tfl1); (II) early-flowering photoperiod-sensitive (elf1 and elf2); and (III) early-flowering, photoperiod-insensitive (elf3). This paper describes the initial phenotypic and genetic analysis of the mutants, and discusses how the genes represented by these mutations may fit into the signal/response pathway that leads to reproductive development.