ABSTRACT In a preliminary study of the oxygen consumption of leeches (Mann, 1956), it was shown that Erpobdella testacea has an oxygen consumption which is independent of the oxygen concentration in the water, over the range 2–6 ml./l., provided that it has been acclimatized overnight to the oxygen concentration of the experiment. The determinations in these experiments were made by enclosing the leeches in glass-stoppered bottles for periods of about 1 hr. Such experiments are unsatisfactory for a study of acclimatization because: (i) the oxygen concentration is constantly changing, (ii) the duration of the experiments is limited, and (iii) the closing of the bottles and the shaking which is necessary to ensure proper mixing of the water make it difficult to keep the animals in a quiescent state. An apparatus was therefore devised to enable the animals to be placed in a slow stream of water of any desired constant oxygen content, so that their oxygen consumption could be measured over a period of days. In this way it was possible to obtain a true resting state, and to investigate acclimatization in differing oxygen concentrations at different times of year.