A side-mounted or vertically-mounted, pleated mosquito screen comprising netting(10) where one side of the netting (10)is attached to and held by a fixed vertical upright (11) while the other side is attached to a mobile vertical upright (12) where the latter is designed to be locked into a further fixed upright (13) opposite to the first upright on the opposite side of the opening to which the mosquito screen is fitted and where the netting (10) is attached to a pre-tensioning device which uses pull wires (16) located in multiple plugs (17) forming a unit which is substantially vertical so that it can be contained inside the track of the fixed upright (13) and also where the upper wire (16) is led out before the end of the mobile upright is reached and is not led out of the end itself.