Experimental manipulations and direct observations were combined to study small-scale dispersion patterns and their causes in an intertidal sand-flat community numerically dominated by sessile and sedentary tube dwellers. Individuals of several species exhibited gregariousness at scales as small as one centimeter. Experiments in which needles were placed in sediments to simulate animal tubes suggested that organisms are affected by patterns of flow which change over distances of several millimeters to a centimeter.