Simulation models are used to examine the possible effects of discrete fuel distributions and of several fire-spread mechanisms on fire shapes. Two postulated fire spread mechanisms -heat accumulation and flame contact-are shown to yield near-ellipses in continuous fuels, but a wide range of shapes in discrete and very patchy fuels. The alternative shapes include ovoids, "tear-drop" (with the ignition point at varying positions on the major axis), and straight lines. Simulated fires in discrete, patchy fuels are less regular in shape than in uniform and continuous fuels and show little or no backburning. The results may explain certain observed differences between wildfire shapes that occur in different environments and at different burning intensities.