A descriptive and quantitative analysis of agonistic signaling and dominance relationships in the Pacific harbor seal (Phoca vitulina richardsi) is presented. Harbor seals rely primarily on the forelimbs and muzzle to convey social signals on land. Communication is achieved by a series of graded responses that vary in intensity depending upon the proximity, orientation, size, and sex of the interactants. Analysis of 1,617 agonistic encounters revealed eight basic patterns: move away, head-up-stare, extended foreflipper, foreflipper wave, foreflipper scratch, growling, closed-mouth head thrust, and open-mouth head thrust. Adult and juvenile seals formed strong linear hierarchies based on size and sex. Dominance ranking among pups is less strongly linear and no clear dominance relationships between male and female pups were evident. Adult males dominated all other age and sex classes, and greatly influenced the dispersion of other groups on land. Juveniles were subordinate to adults, although juvenile males approaching physical maturity dominated some adult females. Disputes between seals were brief; prolonged and aggressive encounters were characteristic of similarly-sized individuals, and may reflect unsettled dominance ranking. Agonistic signaling in phocids exhibiting external features similar to P. vitulina are consistent in form, context, and function to signaling patterns described for P. vitulina in the present study.