The survival of the free-living stages of Trichostrongylus colubriformis under defined conditions of temperature and relative humidity was investigated. The survival of embryonated eggs was poor at 0, 33 and 54.5% relative humidity (rel. hum.) at 20 degrees C but hatching occurred from a proportion of eggs even after exposure for 104 days to 76 and 98% rel. hum. at 20 degrees C. Second-stage larvae were desiccation-susceptible and were killed within 6 h even at 98% rel. hum. and 20 degrees C. Infective larvae, dried separately or in clumps, survived prolonged exposure to desiccation at 33-98% rel. hum. and 20 degrees C with 50% survival times of 58-164 days. Clump formation did not enhance survival in this range. Infective larvae also survived exposure to vacuum desiccation with 50% survival times of 8.8 h in clumps and 4.5 h when dried separately. The infective larva thus readily survives desiccation and may prove a useful model for the study of anhydrobiosis.