Abstract: A portable collapsible ventilator, with a fabric enclosure intended for mounting over a hatch or porthole opening on the deck or hull of marine craft or other similar enclosed spaces. The ventilator consists of a bonnet shaped fabric enclosure which is attached to an opening frame made up of two members hinged at one end. The open ends of the opening frame are held apart at approximately the width of the hatch opening and maintained at that distance by means of a rope or line secured at both ends. The opening frame is made of a resilient material such that when the hinged end is pushed down, the two segments of the opening frame bend outward forming an oval shape and when thus compressed the ventilator may be secured to the deck of the boat or surface of enclosure by means of a pair of lines extending from the hinged point to a retaining bar inserted through and secured under the hatch opening.