Abstract Seven transects of the narrowly-constricted mouth of the Chesapeake Bay, U.S.A., were made using an acoustic doppler current profiler (ADCP) and conductivity-temperature-depth (CTD) sensor during the period 16–21 September 1996. Data were acquired while participating in the Chesapeake Bay Plume Outfall Experiment (COPE I). Seven CTD stations per transect were conducted across the 17 km wide mouth along with continuous high-resolution ADCP coverage. Although discontinuous, the transect data covered nearly all phases of a generic tidal current cycle. The ratio of water level anomaly to average depth was O(10 −3) during the transects with atmospheric pressure and wind accounting for most of the anomaly. Freshwater discharge from the Bay was above normal during this period. The resultant flow structure obtained from the ADCP is much more complete than was previously possible with point measurements. Analysis in all tidal phases revealed near-isothermal conditions with horizontal and vertical temperature differences across the mouth rarely exceeding 1·0 °C in any tidal phase, higher surface salinity to the north coupled with higher subsurface salinity in the Chesapeake Channel and unidirectional flow from surface to depth. Horizontal and vertical current gradients were found to be quite strong, O(10 −3) s −1and O(10 −2) s −1respectively. A consistent feature found on nearly every ADCP transect was a subsurface flood current core and a surface ebb current core, both on the right side of the main channel in the direction of flow.