A monitoring program documented densities of ringed seals, Phoca hispida, before and during development of the Northstar oil field in the central Alaskan Beaufort Sea. Aerial surveys of seals on landfast ice were conducted during the springs of 1997 to 1999 (pre-construction) and 2000–2001 (construction and drilling). Acoustic and vibration data were acquired during the ice-covered seasons of 2000 and 2001. Underwater sounds, in-air sounds, and iceborne vibrations from Northstar were above background levels to distances of 1–5, 0.5–4, and 1–4 km, respectively. However, seal densities close to Northstar in 2000 and 2001 were not reduced relative to those farther away or during the 1997 to 1999 period. © 2003 Acoustical Society of America.