The annual and tidal cycle of behaviour of Uca tangeri (Eydoux, 1835) was studied over a 2 yr period in the Ria Formosa on the Portuguese Algarve coast. U. tangeri settles on the mudflats as well as in the nearby zone of Arthrocnemum spp.. Individuals smaller than 27 mm carapace width live close to the creek; larger crabs inhabit the salt marsh zone. Newly settled crabs are found in the middle of the mudflat. A surface temperature of at least 18 °C is necessary for U. tangeri to become active on the sediment surface. Hence activity shows a clear annual cycle: the months March to July are used for courtship and reproduction. During this time crabs change colour from brown to orange and purple. Large migrations to other mudflats take place from August to October, whereas crabs stay inside their burrows from November to March. During low-tide periods, crab activity can be differentiated into food uptake, courtship, or migration and burrowing.