Distortion-product otoacoustic emissions were recorded from the tympanum of the grasshopper,Locusta migratoria. The hearing organ of this insect is in direct contact with the tympanum and does not contain sensory hair cells.2f1−f2distortions were measured for stimulus frequencies between 2–70 kHz. For frequencies between 3–9 kHz, the level of2f1–f2was 30–50 dB below the stimulus level.2f1−f2threshold curves calculated from distortion growth functions at differentf2frequencies are most sensitive between 3–9 kHz. These thresholds match the auditory sensitivity of low frequency receptor neurons in the ear [Römer, J. Comp. Physiol.109, 101–122 (1976)]. In contrast to vertebrates, the dependence of the2f1−f2level on the frequency ratiof2/f1did not show distinct maxima for mostf2frequencies. The distortion levels were largest for small ratios close to 1. The behavior of2f1−f2was significantly different for stimulus frequencies below and above 10 kHz. Below 10 kHz, the thresholds were more sensitive, the slope of distortion growth curves was shallower by a factor of at least 2, and the distortion levels reversibly decreased duringCO2-induced hypoxia. Nonlinear mechanical processing may therefore be a general feature of sensitive hearing organs, even if these involve very different morphologies. Our results suggest that the the ciliated dendrites of the receptor cells of the insect may play a role in distortion generation.