The man-of-war, although found in warm seas throughout the world, occurs most commonly in the Gulf Stream of the northern Atlantic Ocean and in the tropical and subtropical regions of the Indian and Pacific oceans; it is sometimes found floating in groups of thousands. Physalia physalis is the only widely distributed species. P. utriculus, commonly known as the bluebottle, occurs in the Pacific and Indian oceans.

The body consists of a gas-filled, bladderlike float, which may be 9 to 30 cm (3 to 12 inches) long and may extend 15 cm (6 inches) above the water. It is a translucent structure tinted pink, blue, or violet. Beneath the float are clusters of polyps (polyp), from which hang tentacles of up to 50 metres (about 165 feet) in length. The polyps are of three types: dactylozooid, gonozooid, and gastrozooid, concerned, respectively, with capturing prey, with reproducing, and with feeding.

The animal moves by means of its crest, which functions as a sail. The reproductive habits of Physalia are not fully understood.

Tentacles of the dactylozooids bear nematocysts, stinging structures, that paralyze small fish and other prey. The gastrozooids then attach to the immobilized victim, spread over it, and digest it. The Portuguese man-of-war, in turn, is eaten by other animals, including the loggerhead turtle (Caretta caretta). The fish Nomeus gronovii (man-of-war fish), about 8 cm long, lives among the tentacles of Physalia and is almost immune to the poison from the stinging cells. Nomeus feeds on the tentacles, which are constantly regenerated; sometimes the fish is eaten by Physalia.

The sting of Physalia is very painful to humans and can cause serious effects, including fever, shock, and interference with heart and lung action.