The flow structure associated with a line bubble plume in shallow water is investigated. G. I. Taylor has proposed the use of such plumes as wavebreakers. To be effective the surface current generated should be stable for a distance of the order of the wavelength, which in turn could be several times the depth. It appears that the formation of recirculating cells can affect the wavebreaking potential of line bubble plumes. The paper presents observations and measurements of the cell structure associated with line bubble plumes as obtained in a model towing basin of dimensions 1 × 1 × 40 m. A recirculating region was found on both sides of the plume, but the secondary (and higher-order) cell structures proposed by other investigators were not observed. The primary cells were found to be appreciably longer than those reported in the literature for vertical plane jets. The difference can in part be attributed to different definitions of cell length. The definitions used herein are based on observables, both on the water surface and in the interior of the flow, and they lead to consistent measures of length. Bubble-plume parameters (such as entrainment coefficient) are known to depend on gas flow rate, and it was found that the length of the primary cell is a weak function of this variable as well. Additional experiments with a vertical plane jet were conducted for comparison. Longer cells than those previously reported were again observed. The paper contains a complete theory for line bubble plumes, including the effects of compressibility, bubble slip and finite release volume, as well as a simplified similarity analysis useful in estimating plume properties and horizontal-current depth and velocity.