Many types of cells compensate for induced alterations in the length of one cell cycle phase (G1, S, or G2) by altering the lengths of the other phases. Here we show that, when cells in Drosophila wing discs are delayed in G1, they maintain normal division rates by accelerating passage through S and G2. Similarly, when G2-->M progression is retarded, G1-->S progression accelerates. This compensation mechanism employs negative feedback in which the cyclin-dependent kinases Cdk1 and Cdk2 downregulate the transcription factor dE2F1. dE2F1, in turn, positively regulates cyclin E and string/cdc25, which activate the Cdks to drive cell cycle progression. This homeostatic mechanism coordinates rates of G1-->S and G2-->M progression, maintaining normal rates of proliferation when cell cycle controls are perturbed (e.g., by ectopic Dacapo, dWee1, dMyc, or Rheb). Without dE2F1, the compensatory mechanism fails, and treatments that alter Cdk activity cause aberrant cell cycle timing and cell death.