Adherent cells cultured on flat, homogeneous surfaces typically maintain an intact cell body with a polygonal or fan shape, despite active migration and strong mechanical interactions with the substratum. We hypothesized that, in addition to the constraint of the surface membrane, an active mechanism may be involved in maintaining the shape and integrity of the cell body particularly where cells encounter complex topographic patterns of guidance cues. To detect if there is a mechanism that constrains cell shape, we plated NIH 3T3 fibroblasts on ring-patterned substrata 8-17.5 microns in width and 53-133 microns in outer diameter. Untreated cells have a limited angular span, encompassing an average of 108 degrees around the ring, even though these cells were able to cover a much larger surface when plated on flat surfaces of the same material. Treatment of 3T3 cells with a myosin II inhibitor, blebbistatin, induced a striking increase in the bending ability, causing cells to cover more than 60% of the ring. Inhibition of the Rho-dependent kinase with Y-27632 caused a similar but smaller increase in the angular span. Our results suggest that cell shape is controlled not only by the passive constraint of the surface membrane but also by an active mechanism driven by myosin II-mediated contractility under the regulation of Rho-dependent kinase. The inward surface tension-like forces allow the cell to maintain its integrity while navigating through complex physiological environments.