This paper is prompted by a recent numerical study (Lewin, G. C., and Haj-Hariri, H., "Modelling Thrust Generation of a Two-Dimensional Heaving Airfoil in Viscous Flow,"Journal of Fluid Mechanics, Vol. 492, Oct 2003, pp. 339-362) that shows that for a two-dimensional (2-D) elliptic airfoil undergoing prescribed heaving motion in a viscous fluid, both leading-edge vortices and trailing-edge vortices contributed to the formation of the wake structures. However, an earlier dye-visualization study (Lai, J. C. S., and Platzer, M. F., "Jet Characteristics of a Plunging Airfoil," AIAA Journal, Vol. 37, No. 12, 1999, pp. 1529-1537) on a heaving NACA 0012 airfoil appears to show that the wake structures were derived from trailing-edge vortices only. The dissimilarity in the two studies remains unclear because there is no corresponding experimental data on a 2-D heaving elliptic airfoil. In this study, digital particle image velocimetry technique was used to investigate the wake-structure formation of a 2-D elliptic airfoil undergoing simple harmonic heaving motion. For the range of flow conditions investigated here, our results show that the type of wake structures produced is controlled by when and how the leading-edge vortices interact with the trailing-edge vortices.