Exercise may produce micronuclei (presumably gas-filled bubbles) in tissue, which could serve as nucleation sites for bubbles during subsequent decompression stress. These micronuclei have never been directly detected in humans. Dual-frequency ultrasound (DFU) is a resonance-based, ultrasound technique capable of detecting and sizing small stationary bubbles. We surveyed for bubbles in the legs of 6 normal human subjects (age 28-52) after exercise using DFU. Eleven marked sites on the left thigh and calf were imaged using standard imaging ultrasound. Subjects then rested in a reclining chair for 2 hours prior to exercise. For the hour before exercise a series of baseline measurements were taken at each site using DFU. A minimum of 6 baseline measurements were taken at each site. Subjects exercised at 80% of their age-adjusted maximal heart rate for 30 minutes on an upright bicycle ergometer. After exercise, the subjects returned to the chair and multiple post-exercise measurements were taken at the marked sites. Measurements continued until no further signals consistent with bubbles were returned or one hour had elapsed. All subjects showed signals consistent with bubbles after exercise at at least one site. The percentage of sites in a given subject showing signals significantly greater than baseline (p<0.01) at first measurement ranged from 9.1% to 100%. Overall, 58% of sites showed signals consistent with bubbles at the first post exercise measurement. Signals decreased over time after exercise. These data strongly suggest that exercise produces bubbles detectable using DFU.