Abstract: This invention provides an improved anti-siphon fillneck screen for a vehicle-mounted fuel tanks. The improved device (10) includes an anti-theft member (14) adapted to be removably mounted within a fillneck (13). The anti-theft member has a screen portion (26) and a flange portion (28) extending radially outwardly therefrom. The flange portion is captured between two retaining rings (15,16) inserted into the fillneck. Thus, the improved screen may be removed by means of a special tool.