1. The gustatory effects of miraculin, the sweetness‐inducing protein from the miracle fruit Synsepalum dulcificum, was studied in the rhesus monkey, Macaca mulatta. 2. The intake of five acids was recorded in two‐bottle preference tests, one bottle containing acid and the other tap water, before and after miraculin treatment. All the acids tasted more pleasant after miraculin. 3. The electrical activity of the chorda tympani nerve to stimulation of the tongue with a variety of sweeteners, acids, sodium chloride and quinine hydrochloride was recorded in anaesthetized animals. 4. Pre‐treatment of the tongue with 0·3‐5 mg miraculin doubled the summated nerve response to the acids and diminished the response to sucrose by about 10%. The enhancement lasted for at least an hour and the diminution up to 20 min. 5. After miraculin treatment the Spearman's rank correlation coefficient between the order of increased intake of acids and the order of enhancement of the summated nerve response was 0·99. 6. A solution of 0·1 mg miraculin per ml. elicited a weak nerve response. No preference over water for this concentration of miraculin was recorded in the two‐bottle tests. 7. The activity of twenty‐nine single taste fibres, selected for their responsiveness to sweetness or acids or both, was recorded after miraculin treatment. Effects were obtained in nine fibres which were similar but more pronounced than those observed in the summated recordings. Before miraculin, these fibres responded better and to a larger variety of sweeteners (81%) than the other fibres (40%). After miraculin, acids elicited on the average 2·3 times more activity than before, while the response to sweeteners was depressed. In twenty fibres no effect of miraculin was observed. These fibres responded to fewer of the sweeteners and were more stimulated by the non‐sweet stimuli than the first group. 8. The results suggest that miraculin acts on those structures in the taste cell membrane that are involved in perception of the sweet taste, making them sensitive to acids. The new quality of sweetness after miraculin treatment is signalled by taste fibres which normally respond to sweet substances but which, under the influence of miraculin, are responding to acids. It is likely that the quality of a taste stimulus is conveyed by the identity of the taste fibres.