SYNOPSIS. This is a review of the behavioral, biochemical-physiological, and morphological adaptations of Modiolus demissus (Dillwyn) to the high intertidal habitat. The ribbed mussel, Modiolus, has fully exploited the behavioral adaptations within the limits imposed by the bivalve body plan. Air-gaping is undoubtedly a significant behavioral adaptation which permits aerial respiration and penetration of the high intertidal zone. Modiolus fully utilizes physiological-biochemical adaptations also. The mussel is both eurythermal and euryhaline: the thermal range is at least 56 °C and the salinity range is at least 70%, both impressive ranges for an organism which conforms to these environmental parameters. The two primary adaptations made by Modiolus are an extensive tolerance to dehydration and a very high enzyme thermostability. These two adaptations are resjxmsible for Modiolus' tolerance toward desiccation, variation in salinity, thermal stress, and possibly anaerobic conditions. There is no obvious morphological adaptation. Thus, the physiological adaptations, coupled with air-gaping, are responsible for the vertical penetration of Modiolus into the intertidal zone and its horizontal migration over an extensive geographic range.