This study examined the effects of sensory modality of signals and the background event rate on task induced stress occurring during a vigilance situation. The study subjects were 40 college students (24 men, 16 women) ranging in age from 18 to 39 years. Four experimental conditions were devised with two sensory modalities (audition and vision) combined with two events rates (5 and 40 events/minute). Ten subjects were randomly assigned to each of the four combinations of sensory modality and event rate. All subjects participated in a 50 minute vigil divided into five continuous 10 minute periods of watch. Subjects randomized to the visual modality monitored a video display terminal providing the repetitive presentation of a horizontally oriented white bar. Neutral events were flashes of the bar lasting 247.5 milliseconds (ms) and critical events were flashes lasting 125.0ms. Subjects in the auditory group monitored repetitive bursts of white noise presented binaurally through headphones. Neutral events were bursts lasting 247.5ms and critical events were bursts lasting 200ms. Signal detection declined over time in all combinations of event rate and sensory modality. Detection scores were greater for subjects exposed to a slow event rate as compared with a fast event rate. Stress, determined using activity measurement (restlessness, fidgeting) and Yoshitake's Symptoms of Fatigue scale, was observed in both auditory and visual modalities, demonstrating that stress of sustained attention is not restricted to visual displays. Subjects monitoring the visual display, however, reported greater subjective fatigue and showed greater activity than those monitoring the auditory presentation. The authors conclude that stress arising from activities requiring sustained attention appears to be identified more specifically with the sensory modality of signals than with the event rate context in which they appear.