The paper contains a detailed description of burrow digging by Uca pugilator. Form and length of the burrow may differ widely in individuals. Choice of the spot is determined by many factors but phototaxis and thigmotaxis do not furnish a satisfactory explanation. The end chamber of each burrow is an air chamber used during high tide. The fiddler crab is water-breathing, but can live in air for many weeks without changing the water of its gill chambers. Length of the burrow partly depends on the moisture of the ground. There is no internal periodicity in the life of Uca, all movements invariably depending on external conditions. Each movement is largely adapted to the actual situation.