An actuator device for downhole tools comprising: indexing means for progressively moving a rupturing member (12, 16); a pressure activated actuating member in fluid communication with a chamber; and a frangible member (HA) interposing the indexing means and the chamber, wherein the actuator device is adapted such that, upon a predetermined movement of the rupturing member, the rupturing member ruptures the frangible member to allow high pressure fluid to enter the chamber and activate the actuating member.