IN 1952, Turing proposed a hypothetical molecular mechanism, called the reaction-diffusion system(1), which can develop periodic patterns from an initially homogeneous state. Many theoretical models based on reaction-diffusion have been proposed to account for patterning phenomena in morphogenesis(2-4), but, as yet, there is no conclusive experimental evidence for the existence of such a system in the field of biology(5-8). The marine angelfish, Pomacanthus, has stripe patterns which are not fixed in their skin. Unlike mammal skin patterns, which simply enlarge proportionally during their body growth, the stripes of Pomacanthus maintain the spaces between the lines by the continuous rearrangement of the patterns. Although the pattern alteration varies depending on the conformation of the stripes, a simulation program based on a Turing system can correctly predict future patterns. The striking similarity between the actual and simulated pattern rearrangement strongly suggests that a reaction-diffusion wave is a viable mechanism for the stripe pattern of Pomacanthus.