Beating eukaryotic cilia and flagella produce characteristic complex waveforms that are thought to result from the spatial regulation of the dynein motors that power them. That is, to generate bending, the dynein arms on only a subset of the microtubule doublets within the axoneme (the microtubule assembly within cilia and flagella) are active at any one moment during beating. This theory leads to the prediction, tested by Elizabeth Smith and co-workers on p. 2533, that dynein arms switch between active and inactive forms on specific subsets of doublet microtubules. The authors have combined structural studies with a microtubule-sliding assay in which isolated Chlamydomonas axonemes are partly digested with protease so that flagellar bending becomes a microtubule sliding movement. They show that ATP induces sliding between specific subsets of doublet microtubules and that the pattern of sliding is altered by Ca2+ concentrations that induce a change in the waveform of Chlamydomonas flagella in vivo. These results, conclude the authors, are consistent with the switching hypothesis of axonemal bending.