Division of the cambial cells and their subsequent differentiation into xylem and phloem drives radial expansion of the hypocotyl. Following the transition to reproductive growth, a phase change occurs in the Arabidopsis hypocotyl. During this second phase, the relative rate of xylem production is dramatically increased compared with that of phloem, and xylem fibres that contain thick secondary cell walls also form. Using two different genetic backgrounds and different environmental conditions, we identified a set of core transcriptional changes that is associated with the switch to the second phase of growth in the hypocotyl. Abscisic acid (ABA) signalling pathways are significantly over-represented in this set of core genes. Reverse genetic analysis demonstrated that mutants that are defective in ABA-biosynthesis enzymes exhibited significantly delayed fibre production without affecting the xylem:phloem ratio, and that these effects can be reversed by the application of ABA. The altered morphology is also reflected at the transcript level, with a reduced expression of marker genes that are associated with fibre formation in aba1 mutants. Taken together, the data reveal an essential role for ABA in the regulation of fibre formation.