Summary Twelve species of marine and one fresh-water teleost were studied to determine the reflex and behavioural characteristics of pressure sensitivity and buoyancy adjustment. A pressure gradient chamber was devised, a tube which utilized the sine relationship between angular distance and depth to analyse buoyancy activity. Pinfish and black sea bass were cinephotographically recorded and analysed in detail. All fishes had a yawn reflex subject to ambient pressure changes. It was induced by changes of 2·0 cm H2O or less. Pinfish and sea bass were adapted to the gradient chamber and experimentally analysed for periods from 14 to 46 days. These fishes, within 1–3 days, always established a base reference point, or fix position, around which all buoyancy related activity centred. During critical adjustment it was held for 1–3 min, by a poised fish, with error in three dimensions less than about 2·0 mm. On leaving, the fish regained the fix position with the same precision. These fishes were sensitive to less than 0·5 cm H2O pressure They had reaction time within 0·1 sec to pressure changes. Passive buoyancy displacement induced visual, tactile, and fin orientation reactions which were stereotyped, buoyancy correlated, and proportional to pressure parameters. These reactions included locomotor patterns which were correlated with test-adjust activities in buoyancy compensation. Negative buoyancy was normal at rest, and positive buoyancy was aversive. Physoclistous teleosts are shown to have major behavioural patterns for buoyancy compensation. They can adjust buoyancy by swimbladder regulation with a sensitivity and precision comparable to postural regulation in terrestrial mammals. The behavioural involvement is sufficiently complex and sensitive to account for many practical observations about barometric, tidal, and reproductive behaviour in the natural environment.