All sensory devices, whether biological or artificial, carry appreciable amounts of intrinsic noise. When these internally generated perturbations are sufficiently large, the behaviour of the system is not solely driven by the external stimulus but also by its own spontaneous variability. Behavioural internal noise can be quantified, provided it is expressed in relative units of the noise source externally applied by the stimulus. In humans performing sensory tasks at near threshold performance, the size of internal noise is roughly equivalent to the size of the response fluctuations induced by the external noise source. It is not known how the human estimate compares with other animals, because behavioural internal noise has never been measured in other species. We have adapted the methodology used with humans to the zebrafish, a small teleost that displays robust visually-guided behaviour. Our measurements demonstrate that, under some conditions, it is possible to obtain viable estimates of internal noise in this vertebrate species; the estimates generally fall within the human range, suggesting that the properties of internal noise may reflect general constraints on stimulus–response coupling that apply across animal systems with substantially different characteristics.