Understanding how organisms and organs modulate their size is a fundamental issue that, in plants, also has important agricultural implications. It is known that the switch between proliferation and differentiation plays a crucial role in determining organ size, but how this switch is regulated in plants is unclear. Here, Seung Rhee and colleagues reveal that CHIQUITA 1 (CHIQ1), a gene of unknown function, maintains the transition between proliferation and differentiation in Arabidopsis thaliana. They show that mature organ size in chiq1-1 null mutants is reduced compared with that in wild-type plants, with mature chiq1-1 mutant organs exhibiting fewer and smaller cells. However, during the first week of development, when most growth occurs by cell division, chiq1-1 organs are similar in size to, or bigger than, wild-type organs. By analysing the expression of cell cycle markers and examining cell cycle length, the authors show that CHIQ1 does not compromise the rate of cell cycle progression but instead perturbs the timing of exit from proliferation. In addition, they show that cells in chiq-quad mutants, which lack CHIQ1, CHIQL4, CHIQL5 and CHIQL6, undergo cell differentiation even earlier than those in chiq1-1 mutants. Overall, these findings uncover a role for CHIQ1 and CHIQ-like proteins in maintaining the proliferative capacity of cells and controlling the timing of cell cycle exit during organ development in plants.