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Several experiments were performed to understand the increased efficiency of microsecond-domain pulsed visible laser fragmentation of urinary and biliary stones produced by immersion in water. An optoacoustic sensor and a piezoelectric transducer were used to measure the effect of water immersion on the stress wave traveling through a stone during laser-induced ablation. The stress wave signal increased by a factor of 10 with immersion and was proportional to the square root of laser fluence. The temporal and spatial behavior of the stress wave and cavity (bubble) generated in the surrounding water were examined using pump-probe and microsecond-flash photographic techniques. The results suggest that improved fragmentation efficiency can be explained by confinement of a plasma by water.