Flower colour is determined primarily by the production of pigments, usually anthocyanins or carotenoids, but the shade and intensity of the colour are often changed by other factors such as vacuolar compounds, pH and metal ions(1,2). Pigmentation can also be affected by the shape of epidermal cells, especially those facing prospective pollinators(3,4). A conical shape is believed to increase the proportion of incident light that enters the epidermal cells, enhancing light absorption by the floral pigments, and thus the intensity of their colour. We have identified a gene (mixta) that affects the intensity of pigmentation of epidermal cells in Antirrhinum majus petals. The cells of the corolla lobes fail to differentiate into their normal conical form in mixta mutants. We have cloned the mixta gene by transposon tagging; its sequence reveals that it encodes a Myb-related protein that probably participates in the transcriptional control of epidermal cell shape.