Abstract: A burst plug for positioning in a downhole fluid passage, the burst plug comprising: a shear disc installable in a sealing position in the fluid passage and openable by applying fluid at a burst pressure against the shear disc, a first part formed to retain the shear disc in a fluid flow blocking position after the shear disc is removed from its sealing position and the first part being selected to assume a shear disc retaining position when fluid at burst pressure is in communication therewith and thereafter to assume a second shear disc removal position when fluid in communication therewith is reduced to a selected pressure lower than the burst pressure.