The present study was designed to assess the effects of stimulus certainty and motor task-relevance on auditory transient 40-Hz or gamma band responses. To study the effects of these factors in a balanced design, auditory event-related potentials (ERPs) of 9 young adults were recorded in a passive listening, simple reaction task, and choice-reaction task (target tone probability = 0.5) conditions. Amplitude and phase-locking of event-related gamma activity were analyzed separately at the level of single sweeps by applying a method that allows the independent quantification of phase synchronization between consecutive single responses. Major results demonstrated that (1) During auditory stimulus processing discernible gamma oscillation bursts were observed in three time windows of the poststimulus epoch: early (0-120) ms, middle (120-250 ms) and late (250-400 ms). (2) Early gamma response amplitudes were significantly largest for highly expected motor-task stimuli, whereas the phase-locking did not depend on either of the two variables. (3) The phase-locking of late gamma responses, however, was significantly stronger to targets than to nontargets. These results indicate that auditory gamma responses are functionally involved in the processing of task variables such as stimulus certainty and motor-task relevance. It is also demonstrated that single gamma response amplitude and phase-locking have independent functional significance as being affected in a different manner by different task conditions.