The streamwise and spanwise evolutions of finite span synthetic (zero net mass flux) jets were investigated experimentally using particle image velocimetry. The synthetic jet was produced over a broad range of length and time scales at Reynolds numbers from 85 to 364, stroke lengths from 16 to 50 times the slit width, and two formation frequencies, f =300 and 917Hz. The velocity and vorticity fields were measured in two planes, across the slit (i.e., along the short axis of the orifice) and along the slit (i.e., along the long axis). The paper presents the effect of the slit aspect ratio on the development of the synthetic jet, and the spatial evolution of secondary three-dimensional vortical structures in the flow field. The measurements in the plane along the slit revealed a unique flow pattern, where near the orifice the flow is two-dimensional, while farther downstream the vortex pair lines develop secondary counter-rotating structures. The streamwise and spanwise spacing between these structures vary with stroke length and formation frequency. As the orifice aspect ratio increases the effect of the slit edges decreases, thus the secondary structures are less pronounced.