Survivorship of cercariae of Postharmostomum helicis (Trematoda: Brachylaimidae) was examined under different temperatures and relative humidities. Cercariae were isolated from naturally infected snails (Anguispira alternata) and maintained at combinations of 3 temperatures (16.6, 22.2, and 29.4°C) and 5 relative humidities (0%, 25%, 50%, 75%, and 100%). Cercariae maintained at low relative humidities (0–50%) died rapidly at all temperatures, surviving 30 min or less on average. At higher relative humidities, life span extended for 3 or more hr. Vapor pressure deficit was a good predictor of life span except for trials at 100% relative humidity, wherein cercariae often died prior to desiccation of the mucous trails in which they were shed. The results of the present investigation suggest that life span is highly limited except under the moistest conditions.