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In natural waters the dissolved oxygen concentration can vary considerably both spatially and temporally in rivers (Solbé, 1988), lakes (Gnaiger, 1983), coral reefs (Griffith et al. 1987) and the thermocline in the open ocean (Dizon et al. 1987). There are uncertainties in extrapolating laboratory studies on effects of hypoxia on fish performance (Jones, 1971) to field situations (Priede et al. 1988). This paper describes a transmitter which enables a continuous readout of fluctuations experienced by fish during normal activity in such conditions to be monitored by a remote receiving station on the shore or in a boat.