The relationship between thallus shape and habitat inclination in the umbilicate lichen Lasallia pustulata is investigated. There is a significant relationship between the steepness of the habitat rock surface and the degree of asymmetry around the central holdfast of the thalli. This is interpreted as an effect of gravity, including the gravity-influenced over-under 'tiling' of the lichens on the habitat surface. There was no correlation between thallus size and degree of asymmetry.