Observations were made on the penetration of human skin by cercariae of Trichobilharzia stagnicolae, the chief cause of schistosome dermatitis in the United States. It was found that the cercariae will penetrate while fully submerged and, therefore, do not require an evaporating film of water to stimulate or aid penetration. In some tests the cercariae penetrated from relatively cool water (17 C to 23.5 C), the temperature of which changed little during the exposure period. It was also found that the itching associated with penetration of the cercariae occurs very soon after the cercarial body enters the outermost layer of the skin and that, therefore, this primary itching may be taken as a reliable indicator of the time of penetration of the cercariae. Primary itching was found to begin from 4 to 10 or more minutes after the cercariae were placed on the skin, indicating that the cercariae remain on the surface of the skin for some time before penetrating.