Studies of the activity of accessory respiratory muscles were conducted in 30 normal subjects (15 male and 15 female subjects, age 5 to 62 years). Electromyograms were recorded over the right serratus anterior muscle and the adjacent sixthe intercostal space, and the results were correlated with tidal volume. Phasic respiratory activity in the serratus anterior muscle was detected in every subject but was markedly affected by the prevailing posture and level of ventilation (appearing at mean tidal volumes equivalent to 60.5 percent of vital capacity when subjects were standing relaxed, and equivalent to 35.2 percent of vital capacity when subjects were seated, leaning forward with elbows supported). The activity was predominantly inspiratory in timing but in some subjects extended into early expiration. No significant differences were noted in subjects of different ages, sex, height, or weight. Similar results were recorded from the inspiratory intercostal muscle. On the basis of these findings, we conclude that, contrary to reports, the serratus anterior muscle should be classified as an accessory muscle of respiration, at least at increased levels of ventilation, and that its activity is most pronounced in postures that place the muscles' origin in an advantageous position for moving the ribs.