Zebrafish display a prominent pattern of alternating dark and light stripes generated by the precise positioning of pigment cells in the skin. This arrangement is the result of coordinated cell movements, cell shape changes and the organisation of pigment cells during metamorphosis. Iridophores play a crucial part in this process by switching between the dense form of the light stripes and the loose form of the dark stripes. Adult schachbrett (sbr) mutants exhibit delayed changes in iridophore shape and organisation caused by truncations in Tight Junction Protein 1a (ZO-1a). In sbr mutants, the dark stripes are interrupted by dense iridophores invading as coherent sheets. Immuno-labelling and chimeric analyses indicate that Tjp1a is expressed in dense iridophores, but down-regulated in the loose form. Tjp-1a is a novel regulator of cell shape changes during colour pattern formation and the first cytoplasmic protein implicated in this process.