Burrowing activities of the only European fiddler crab, Uca tangeri, and its resulting influence on biotope in mudflats were investigated during 1989–1990 at Ria Formosa, Portugal. Individuals use the same burrow for ca 1 wk, then occupy another or dig a new one. Overall a burrow is inhabited for ca 3 month by several individuals before it is abandoned. Vacated burrows decay within 2 to 3 wk. Burrow size and number vary with the season. Burrow density was highest in spring and early summer with ca 17 burrows m–2, and then decreased. Deepest burrows (up to 90 cm long) were found in winter, the shallowest (up to 40 cm long) in summer. Volume of the sediment moved by U. tangeri varied monthly between 3000 and 6000 cm3 per m2 of mudflat. Water is only found in the lower third of the burrow. Burrow water contains less oxygen and more nitrate than the surrounding water of the Ria Formosa.