Activity is both diurnal and nocturnal. Olfaction may be important in communicating intraspecific information. Phocids defend themselves by opening their mouth, uttering menacing cries, and advancing against the enemy, or they may flee to the water and dive. Most species eat fish, shellfish, and cephalopods. Hydrurga is the only pinniped that preys regularly on penguins and other seals.
Some species can fast for long periods.
Phocids do not usually congregate in large rookeries as do otariids. Males may establish territories on land during the breeding season. Most species live in pairs during this period, but the males of some species, such as Halichoerus and Mirounga, may mate with a number of females. The total time of pregnancy is 270-350 days and usually includes a period of delayed implantation. At birth the single pup weighs 5-40 kg.