1. 1. More than 80% of the body water in Balanus balanoides was frozen at the winter median lethal temperature (MLT—18.6°C). This confirms the suggestion made previously that the mechanism of cold-tolerance in B. balanoides is due to frost resistance rather than avoiding ice formation. 2. 2. In the summer only 40–45% of the body water of B. balanoides was frozen at the summer MLT (−6.6°C). This is very different from the situation envisaged by Williams (1970) for intertidal molluscs. where a constant proportion of the body water is frozen at the lower lethal temperature.