The colour patterns of live fiddler crabs and ontogenetic colour changes were documented in the field and compared with morphological results (particularly the shape of the male gonopods) obtained from a new collection. According to these studies the fiddler crabs of Darwin consist of nine species: Uca capricomis Crane, U. dampieri Crane , U. elegans George and Jones, U. flammula Crane, U. hirsutimanus George and Jones, U. mjoebergi Rathbun, U. polita Crane, U. seismella Crane, U. signata (Hess). The presence of additional Uca species, viz. U. coarctata (H. Milne Edwards), U. dussumieri (H. Milne Edwards) and U. vomeris McNeill reported by George and Jones ( 1982), is not confirmed here. These species appear to be confined to the east coast of Australia. It is also concluded that U. pavo George and Jones is a synonym of U. capricornis Crane: U. pavo represents an adult colour stage which was not adequately described in the original publication of Crane.