Examination of the downflow width behavior of 59 terrestrial lava flows at Puu Oo (Hawaii) and Glass Mountain (California) and 86 Martian flows at Alba Patera, Tyrrhena Patera, Elysium, and Olympus Mons was completed using aerial photographs, topographic maps, previously published flow maps, and Viking Orbiter images. The examined lava flows exhibit diverse width behavior, from which information about flow processes and conditions was assessed. For Puu Oo flows, no significant correlation was found between the average width of a flow and flow length or average underlying slope. A significant, but weak relationship was found between average width and average flow thickness. In analyses of the downflow width behavior of individual flows, no consistent correlations were observed between width and thickness or underlying slope. When width was analyzed as a function of distance from the source for all flows, a variety of flow width behavioral trends were recognized and quantitatively classified. The most common behavior observed on Earth and Mars involved variations of width (sometimes significant) about a mean without a significant downflow narrowing or widening trend. The distributions of width behavior trends for the Alba Patera and Puu Oo flows examined were similar, with this type of constant behavior dominating. In contrast, Tyrrhena Patera flows showed a tendency to widen with distance downflow, and silicic flows at Glass Mountain were more likely to narrow. Flows were also subdivided by distance from the vent, and the width behavior of each division classified. Subdivision of flows resulted in significant changes in the classification of width behavior. While width behavior in the medial regions of flows was similar to that over entire flow lengths, proximal regions show more variability (possibly due to greater fluidity of lavas near the vent) and distal regions tend to uniformly narrow (possibly due to limited supply). In certain cases, classification and subdivision analysis can be used to link flow emplacement processes to the resulting morphology. In particular, width behavior can be correlated to the presence or absence of lateral levees. ¿ 1999 American Geophysical Union