Abstract.This study of the enteropneust,Balanoglossus aurantiacus, in Bogue Sound, NC, provides information on the general structure and variability in geometry of its burrow,B. aurantiacusemploys a funnel at the anterior end of its burrow to cave in surface sediments. It ingests, processes, then deposits these sediments back on the surface as fecal castings. Measurements made on resin casts of burrows and on photographs of surface features (burrow openings and funnels) revealed a new and consistent pattern of burrow construction. This pattern distinguishesB. aurantiacusburrows from those reported for other funnel feeders and enteropneusts (except, perhaps, forB. clavigerus).Laboratory and field observations revealed that, within its burrow,B. aurantiacusis elongated, can double‐back on itself, and can use anterior burrow openings for subsequent fecal castings. Tests for significant changes in location of surface features gave estimates of how frequently individuals in aquaria and field populations abandon old burrow locations and construct new burrows. Field observations showed thatB. aurantiacuswill reopen a blocked burrow openin