Superhydration of the horny layer by water-soaked cotton felt pads applied under continuous occlusion for seven days produced a vesiculopustular rash in 26 of 32 trials. This did not happen when occlusion was secured by impermeable plastic film alone. Although with superhydration various gram-negative bacilli became established, there was a compelling correlation with high surface levels of Pseudomonas. The severity of the rash was proportionate to the quantity of Pseudomonas, the density generally exceeding 0.5 x 10 6 /sq cm. Organisms were not found within the skin. The histopathologic findings were typical of a toxic reaction: epidermal necrosis, hemorrhage, and massive infiltration by neutrophils.