1. The ensheathed larva of Haemonchus contortus is far better at surviving desiccation than the exsheathed form.
2. Using interference microscopy, it is shown that, although the sheath is freely permeable to water, the ensheathed larva loses water much more slowly than the exsheathed larva.
3. Close inspection of partially dry ensheathed larvae shows that the sheath becomes dry first and that this slows the rate at which the larva it contains loses water.
4. The results are discussed in relation to other examples among nematodes where desiccation survival appears to be associated with mechanisms to slow the rate of drying.