Abstract: A closure device for mounting on an opening of a receptacle containing a fluid, said device comprising: a base (1) comprising fastener means (12) for fastening the base onto the opening of the receptacle, said base defining a fluid-dispensing orifice (11) through which the fluid contained in the receptacle can be dispensed; and an openable cap (2) for mounting on the base (1), said cap including closure means (210) for closing the dispenser orifice (11) of the base (1); the device further comprising first-use indicator means (25) for making it easy to see whether the cap (2) has already been withdrawn from the base (1), the device being characterized in that said indicator means comprise a visual indicator member (25) that is displaceable between an initial position that guarantees that the cap has not already been opened, and a final position that indicates that the cap has already been opened, said indicator member (25) reaching its final position when the cap (2) is put back into place on its base (1)