SYNOPSIS. A critically heat-stressed population of Modiolus modiolus has been studied in a tidal pool. Over a period of 5.5 hours the temperature in the shallows rose from 19°C to 32.5°C. Some of the animals were killed outright; others succumbed over the next few days. Most of the animals, although distressed at the time of exposure, seeined to have recovered by the following week. The animals most adversely affected were those whose shells protruded above the surface of the pool; these individuals gaped and became desiccated. The reactions of the species are consistent with previous hysiological experiments. High temperature plays a role in limiting the vertical distribution of Modiolus modiolus.