The subtidal bivalve Glycymeris glycymeris (L.) exhibits a high degree of respiratory independence in conditions of declining environmental oxygen tension. In contrast to other bivalves previously studied, the index of respiratory independence,K1K2decreases with increasing weight specific oxygen consumption indicating that small Glycymeris are better regulators of oxygen consumption than large Glycymeris. The respiratory responses of Glycymeris to hypoxia include a small initial increase in ventilation, brought about by increasing the percentage of time spent pumping and a large increase in oxygen utilization. Heart activity is elevated, principally through a large increase in the amplitude of heart beat, which suggests increased perfusion of the respiratory surfaces. The ventilation : relative perfusion ratio, therefore, declines over the range of oxygen tension that respiratory independence is maintained. The respiratory mechanism of Glycymeris is compared with that previously described for other bivalves and it is concluded that there are no clearcut differences between the respiratory responses to hypoxia of intertidal and subtidal species.