Juveniles of the common seal, Phoca vitulina, have two kinds of aquatic play : (1) Dyadic play, in which muzzle-to-body and body contact between two animals is combined with exuberant somersaulting movements. The somersaulting behaviour is usually preceded by a contact phase with very little movement. A play bout may end with a period of sustained, almost static contact. (2) Group play, which resembles the normal social haul-out activities, but each activity is repeated several times in a playful manner. Although several animals may be leaping and splashing simultaneously, each animal temporarily orients his play towards one other, whom it may contact briefly. All play by juveniles of the grey seal, Halichoerus grypus, is preceded by each animal of the dyad giving a stereotyped invitation signal, which must be repeated continually by both animals throughout play : each animal in turn lays its head over its partner's back. For the initial phase of play which takes place on the beach, the two animals lie beside one another, each lunging gently at the other's head, in between head-over-back signals. Adolescents may occasionally rear up and lunge at each other in a manner similar to the fighting of adult males. After playing on the beach, the pair may enter the water, where their play is similar to the dyadic play of the common seal. The aquatic somersaulting over one another by two animals in continuous body contact seems to be a pattern not found in these two species in other functional contexts. Common seal group play serves to (i) integrate the individuals into a unified group, and (ii) acquaint individuals. Although grey seals apparently do not have group play, behaviour which is probably functionally analogous was observed, in which the seals became acquainted over a 3-week period in early autumn before any play occurred.