ABSTRACT-Under laboratory conditions swimming behavior of larvae of the infaunal bivalve Sinonovacula constricta was dominated by a vertically oriented helical spiral with intermittent sinking, enabling larvae to regulate their vertical position in the water column. Instantaneous swimming velocities ranged between 0.4 and 0.7 mm S-' for veliger and veliconcha larvae; these values were directly comparable to those recorded for other marine bivalve larvae. Swimming activity d u e to ciliary beating in the velum represented a negligible fraction of total energy expenditure. Postlarval drifting was first observed on Day 9 after larval metamorphosis and settlement and continued for about 1 mo. A major mechanism underlying postlarval drifting was the secretion of a single byssus thread which was at least 50 times longer than the body size. Calculations Indicated that the viscous drag of the elongated thread was about 6.7 times greater than the viscous drag of the body, thus providing a hydrodynamic basis for postlarvae to remain suspended in the water column. Environmental factors that increased postlarval drifting included lowered temperature, starvation and decreased light ~llumination. There were at least 3 peaks of postlarval drifting, especially on Day 26 after the larval settlement, these data were consistent with direct field observations. Combination of both physical (e.g. tidal current transport) and biological (e.g. byssus thread) processes are critical in determining the secondary pelagic drifting of S. constricta postlarvae.