Motile cilia on ciliated epithelial cells generate fluid flow in the brain ventricles, trachea and oviduct, which is essential for the proper function of these organs. Fluid flow is a function of ciliary length, but the molecular mechanisms that regulate the optimal length of motile cilia in mammals are unclear. This study shows an important role for the kinesin 8 family member KIF19A in preventing abnormally long cilia. Kif19a−/− mice had hydrocephalus, and female Kif19a-knockout mice were infertile owing to oviduct obstruction. This was associated with increased ciliary length on all ciliated epithelial cells, which resulted in abnormal ciliary waveforms and an inability to generate proper fluid flow. KIF19A was shown to be a ciliary tip protein that negatively regulates the length of microtubules polymerized from axonemes (which form the cytoskeleton of motile cilia) by causing ATP-dependent depolymerization from the plus end.