Between 15 March and 29 July 1998 we quantified aggressive movements (AMs) in a breeding colony of 44 adult magellanic penguins (MPs) during their nesting season at San Francisco Zoo (SFZ). We sampled their behavior for a total of 149.5 hrs in 46 sessions of 195 min each, all starting at 0945 hrs and stopping at 1300 hrs. We recognized 4 types of AMs, of which 2 (eye-pointing and charging) were threat signals, I (bill-fencing) was a ritualized contest of strength and agility, and I (biting) was physical attack. MPs only performed AMs within 2-3 m of nest entrances. Males performed 83% of all AMs, females 17%. The eye-pointing threat signal accounted for 92% of all AMs, and the 3 non-attack AMs combined accounted for 99% of all AMs; only I% involved physical assault. Both sexes performed eye-pointing and bill-fencing, but only males charged or bit their adversaries.