An interlocking intumescent collar is disclosed. The collar may be fitted around a pipe at the point where the pipe penetrates a wall or floor. The collar comprises a strip of intumescent material disposed on a collar body. Anchor tabs extend outward from the collar body and are adapted to be fastened to the wall or floor. The collar includes an interlock integral with the anchor tabs. The interlock holds the tabs in overlapping relationship when the collar is placed around a pipe. The interlock integral with the anchor tabs decreases manufacturing costs and facilitates the field installation of the collar.