Environmental stresses, such as nutrient fluctuations, can affect developmental progression in animals. C. elegans larvae, for example, normally develop into adults through four larval stages under the control of heterochronic (developmental timing) genes such as lin-42, a homologue of the circadian rhythm gene period. But, when times are hard, C. elegans forms long-lived dauer larvae, an alternative third larval stage. Now, Ann Rougvie and co-workers report that lin-42 functions in dauer entry (see p. 3501). Loss of lin-42, they report, makes animals hypersensitive to dauer formation under stressful conditions, whereas misexpression of lin-42 in pre-dauer stages inhibits dauer formation. Other experiments suggest that LIN-42 acts in opposition to the ligand-free form of the nuclear receptor DAF-12, which integrates external cues and developmental decisions. Together, these results suggest that LIN-42 and DAF-12 are intimate partners in the decision to become a dauer larva and raise the possibility that Period-like proteins play a conserved role in coordinating intrinsic timing mechanisms with environmental conditions.