Maintaining the Portuguese man-of-war Physalia physalis in captivity, either for display or study, has not been successful to date. A new aquarium was designed and constructed in an attempt to simulate natural conditions so more could be learned about the husbandry requirements of this species. The prototype aquarium increased longevity, up to four to seven times that of previous attempts, and promoted natural behaviours. Inside the aquarium six acrylic containment cylinders, each containing one Portuguese man-of-war, held the animals away from the sides and the force of water being drawn down through the cylinders retained the centred position of the specimens. Detailed information is given on the design of the aquarium in relation to the natural behaviour of the Portuguese man-of-war.