A gassing incubator for cultivating human or animal cells or tissues is known that has an inner case which can be closed by a door and is surrounded by a thermally insulating outer case. The outer case is spaced away from the side walls, back wall and roof of the inner case. Electrical heaters are disposed under the floor of the inner case and in the area of the side walls. The incubator has a means for humidifying the inner chamber atmosphere. To improved such an incubator so as to achieve a high relative humidity in the test chamber, and a rapid and uniform heating of the water, and to substantially prevent the formation of condensate on the inner wall of the case and enable the heating system to be made at low cost, the electrical heating elements are tubular heating elements which lie flat directly against the underside of the floor of the inner case, the humidifier is a pan containing water, and the tubular heating elements are bent upwardly at the side walls and back wall of the inner case such that they reach above the floor level of the inner case by the depth of the water bath, thus heating the transition area between the floor and the side walls and back wall.