In the cochlear epithelium, the precise morphological organisation of hair cells along the proximo-distal axis is crucial for our sense of hearing: high frequency-responsive hair cells at the proximal end of the cochlea have an expanded apical surface area compared with the low frequency-responsive cells at the distal end. How is this phenomenon, known as tonotopy, established during development? Zoë Mann and colleagues have previously shown that, in chick, an interplay between metabolic and morphogen gradients regulates positional identity. Now, they investigate the role of mitochondrial dynamics in determining hair cell morphology along the proximo-distal axis. Having identified the window during development when the asymmetry in cell morphology is established, the authors show that, during this window, the mitochondrial network grows in size and complexity more in the proximal cells than in the distal cells. Inhibiting mitochondrial fusion using pharmacological inhibitors leads to a reduction in the size of the apical surface area of proximal cells - suggesting that mitochondrial dynamics play a causal role in defining cell morphology along the proximo-distal axis. Thus, this work demonstrates the importance of regulated mitochondrial dynamics in determining cell morphology and, hence, developmental patterning and ultimately tissue function.