Direct observations with 12 moored current meters of the velocity field in the Tiran Strait has confirmed the presence of a well-developed two-layered gravitational convection circulation between the Red Sea and the Gulf of Aqaba. Measurements indicate a level of no motion at 100 m in the deep (270 m) Enterprise Passage, whereas the shallower Grafton Passage (80 m) is completely in the upper layer. Characteristic speeds are 30 cm/sec and 60 cm/sec in the upper and lower layers, respectively. Integration over the channel area indicates an average discharge in each layer of ∼29,000 m3/sec for February 1982. Evidence is presented that the volume balance is quasi-steady over a 2–3-day time scale. Using a monthly evaporation rate, volume and salt conservation equations predict a discharge within 10–15% of observed values. The Enterprise Passage shows clear evidence of reaching the critical flow state of Stommel and Farmer (1952) and Bryden and Stommel (1983). Periodic episodes of critical flow are controlled by the spring-neap tidal current cycle. During neap tides, when turbulent mixing is at a minimum, stratification strengthens and gravitational convection spins up. The strong northerly winds (10–15 m/sec) exert surprisingly little effect on the mean flow. It is anticipated that higher summer evaporation rates will increase the duration and frequency of critical flow.