Animals that are warned about the presence of a predator are more likely to avoid and/or survive an encounter with a predator. Chemical signals released by disturbed or injured conspecifics may provide prey animals with an early warning. In this study we conducted experiments to determine whether larval red-legged frogs respond to chemical stimuli produced by disturbed conspecifics and to examine the chemical compounds that may act as the alarm signal. In laboratory tests, groups of tadpoles responded with antipredator behaviours when exposed to chemical cues of disturbed conspecifics but not when exposed to chemical cues of control (undisturbed) conspecifics. In subsequent tests, disturbed animals increased ammonium (the main metabolic waste of tadpoles) excretion relative to undisturbed individuals. When tadpoles were exposed to low-level ammonium concentrations (1mg NH₄⁺/litre), they responded by increasing antipredator behaviours. Our results suggest that red-legged frog tadpoles release a chemical that provides conspecifics with an early warning of predator presence, and that ammonium (NH₄⁺) may be a component of the disturbance signal.