The shape and motion of the pectoral fins vary considerably among fishes that swim in the labriform mode. Pectoral fin motion in fishes is highly variable, but one conspicuous axis of this variation is the rowing-flapping axis. At one extreme of this axis, paddle-shaped fins row back and forth in a plane that is parallel to fish motion, while at the other extreme, wing-shaped fins flap up and down in a plane that is perpendicular to fish motion. We have used two fish, the threespine stickleback (Gasterosteus aculeatus) and the bird wrasse (Gomphosus varius), that fall near the extremes of the rowing-flapping axis to study the dynamic, energetic, and ecological and evolutionary consequences of this kinematic variation. Our work confirms some traditionally held assumptions about rowing and flapping dynamics and energetics but reject others. A computer simulation experiment of virtual rowing and flapping appendages makes several predictions about differences in maneuvering performance and swimming energetics between rowing and flapping, which, in turn, make predictions about the behavior and ecological distribution of fishes that vary along the rowing-flapping axis. Both laboratory and field studies of labrid swimming ability and distribution support these predictions.