Surgical stapler with anvil sensor and lockout

A surgical stapler is provided for applying linear parallel rows of staples. The stapler includes an anvil retention portion, a cartridge retention portion, a replaceable anvil, a cartridge housing and a blocking member. The blocking member is movable between a free-movement position and a blocking position. In the free-movement position, the blocking member permits movement of the cartridge retention portion and the anvil retention portion between open and closed positions. In the blocking position, the blocking member prevents movement of the anvil retention portion and the cartridge retention portion between the open and closed positions.