A behavioural alarm reaction was shown by 78% of the non‐operated juvenile matrinxã__Brycon cephalus__ exposed to a conspecific skin extract, whereas the other 22% did not change their behaviour. The main reaction (72% of those reacting) consisted of a brief initial phase (2 min) of dashing or very rapid swimming followed by a long‐lasting period of immobility (up to 30 min). The second most frequent behaviour (22% of the reacting fish) was immobility, with the animals becoming motionless immediately after exposure to the conspecific skin extract, for periods ranging from 5 to 30 min. Only one animal slowed its swimming activity. The responses, evaluated by the number of times the animal crossed the grid in the rear of the aquarium, were significantly lower when compared to the baseline swimming activity. Histological examination of matrinxã epidermis revealed the existence of club cells. Complete sectioning of the olfactory tracts as well as bilateral sections of the olfactory sub‐tracts (medial or lateral), stopped the alarm reaction, confirming the importance of olfaction in the ability to recognize the chemical stimulus. Despite the behavioural responses, no variation was detected in blood glucose or cortisol levels, a result that is discussed in terms of the intensity of the disturbance caused by the alarm substance.