Metachronal rowing is a ubiquitous locomotion method among organisms with multiple in-line appendages in which posterior appendages start the stroke and are then followed by more anterior neighbors. Organisms relying on this swimming technique range in size from ciliates to lobsters. The leg kinematics of metachronally swimming organisms have been previously reported for a wide range of swimming speeds. However, the appendage hydrodynamics of freely swimming animals have only been measured for hovering or slowly swimming animals. Here we present time-resolved 2D PIV measurements of the flow generated by a swimming mantis shrimp (Odontodactylus scyllarus) filmed at up to 1000 Hz using a near-infrared laser. Flow field measurements are acquired in sagittal, near-frontal, and transverse planes on an animal with body and pleopod lengths of 114 mm and 15 mm, respectively. Pleopod kinematics also are measured from the sagittal recordings. The mantis shrimp swims at speeds of 0.2-1.9 m/s by beating its pleopods at 3.6-12.5 Hz, which correspond to advance ratios of 1.1-1.5. Measurements in the sagittal plane show that each stroking pleopod pair creates a backwards-moving vortex which evades destruction by the recovery strokes of the other pleopod pairs. The vortex created by the anteriormost pleopod pair is the strongest, and owing to the high advance ratio, the posteriormost pleopod pair lies above this vortex and sweeps into it during its power stroke. As a result, the strength of this vortex increases, an interaction which may increase swimming speed or efficiency. Finally, flow measurements in the near-frontal plane show a pulsed, backward-directed jet accompanied by counter-rotating vortices resembling a reverse von Karman vortex street in the animal’s wake.