Captive alligators (Alligator mississipiensis) emitted short grunts and, lees frequently in the smaller ones, hissing sounds. One [male] alligator, probably over 5 yrs. of age, was accidentally stimulated to roar by the plucking of a steel rod. The effective interval between plucks was 5 to 10 sec.; longer intervals or continuous sound did not produce response. Changes in position of the alligator during roaring are described and illustrated. The roar lasted approx. 1 sec.; it began on a low note, rose in a crescendo, and then both pitch and volume decreased gradually. When roars were successively produced, the intra-roar interval lasted from 4.5 to 6.5 sec. The number of consecutive roars was usually from 6 to 8. Other responses to the auditory stimulus included expansion of the pupil, movement toward the source of sound, sometimes involving the "aggressive approach" posture, and lashing of the tail. In addition, a smaller c? was attacked repeatedly when placed with the roaring alligator. Visual stimuli were eliminated in an expt. showing that the alligator''s localization of the source of sound was a purely auditory response. The alligator roared as readily in a drained tank as in one with 8 inches of water. Complete auditory responses were provoked only by frequencies near those of the reptile''s own roar (57 vibrations per sec.) and the first harmonic thereof. The effective range of auditory stimulation was apparently between 57 and 341 vibrations per sec. These findings were confirmed by using for sound sources musical instruments and recordings of the vibrations of the steel rod and of the reptile''s own roar. Response by a 2d alligator of the same sp. to a French horn blown at about 57 vibrations per sec. at 5 sec. intervals included the roaring, eye, postural, and locomotor reactions. The alligator''s vocalization and response to auditory stimulation may be correlated with inner ear structure. Under natural conditions, roars are apt to induce aggressive behavior in other ?.