All free-living stages of the nematode parasite of sheep, Trichostrongylus colubriformis Giles, survived exposure to freezing temperatures in contact with water, with the exception of the first-stage juvenile (J1). The third-stage juvenile (J3) was the most resistant stage. The order of relative survival of the different stages was different from that of the lowest F50 (the temperature at which 50% froze), suggesting that an ability to supercool was not the only determinant of survival. 2. The F50 was shown to be a good measure of the degree of supercooling and to extend greatly the lower size limit of organisms that could be measured. 3. The J3 uses a freeze-avoiding strategy by supercooling when in air or covered by liquid paraffin. In water it uses a mixture of freeze-avoiding and freeze-tolerant strategies, with a proportion of the population surviving freezing caused by exogenous ice nucleation. 4. Removal of the J3 sheath results in a shift from freeze avoidance to freeze tolerance, with an overall reduction in survival. A major function of the sheath may be to reduce the probability of exogenous ice nucleation.
