Predation can shape virtually every aspect of an organism’s behavior, life history, and evolution (e.g., Dill 1987; Godin 1995; Reznick et al. 1996). Thus, understanding how organisms respond to perceived predation risk or the actual presence of predators is central to understanding their foraging decisions, habitat use, reproductive effort, and other proximate behaviors (e.g., Sih 1982; Ydenberg and Dill 1986; Gillam and Fraser 1987; Heithaus and Frid 2003). For harbor seals (Phoca vitulina richardii), predation (or the risk of predation) is thought to be common enough to influence habitat utilization (Calambokidis et al. 1987; Nordstrom 2002). Harbor seals constitute one of the primary prey species of transient killer whales (Orcinus orca) in the North Pacific (Ford et al. 1998; Saulitis et al. 2000; Heise et al. 2003) and must balance foraging decisions with exposure to killer whales (Frid et al. 2006). There are numerous records of killer whales attacking and killing harbor seals; however, most observations have been analyzed in terms of the predatory behavior of killer whales (Baird and Dill 1996; Ford et al. 1998; Saulitis et al. 2000), with a few exceptions (Deecke et al. 2002). Herein we present data on dive and antipredator behavior by a harbor seal in the presence of transient killer whales. Our observation was unique in that the harbor seal was instrumented with both a VHF headmount transmitter (MM340, Advanced Telemetry Systems, Inc., Isanti, MN) and a timedepth recorder (TDR) (Mk9, Wildlife Computers, Redmond, WA) thus, providing us with dive and location data before, during, and after the harbor seal’s encounter with the transient killer whales. As part of a study examining the foraging ecology of harbor seals in Glacier Bay National Park (GBNP), we observed apparent antipredator behavior by a radio-tagged harbor seal when transient killer whales arrived in the area near the seal. GBNP has