Sexual differences in receptor tuning are found in the auditory system in at least two species of anuran amphibians and are common in electric fish. Since the boundary of a communication signal's active space is defined in terms of the sensitivity of the receptors to that signal, such sexual differences might result in active-space differences between males and females. In spring peepers, the sexual difference in the auditory system clearly results in a significantly larger active space in females. Among electric fish, however, sexual differences in the spectral tuning of the electroreceptors may not always result in sexual differences in active space.