Abstract 1. 1. An ecological and behavioural study was made of the five Uca species of Inhaca Island. 2. 2. In addition to the claw-waving shown by males as a threat-display or as part of courtship behaviour, various modifications of the phenomenon of claw-waving were also noticed. The most widespread of these modifications was a type of synchronous claw-waving, shown only by males of U. annulipes. 3. 3. Synchronous claw-waving is a form of concerted waving by small groups of individuals. 4. 4. The movements involved in synchronous claw-waving are described in detail. 5. 5. Synchronous claw-waving showed both a seasonal and a diurnal peak, and thus has a definite connection with temperature. 6. 6. This highly specialised form of claw-waving is probably due to the abundant energy of the U. annulipes individuals, which spend the most time of all the species above-ground, feeding.