A fork instruction for execution on a multithreaded microprocessor and occupying a single instruction issue slot is disclosed. The fork instruction, executing in a parent thread, includes a first operand specifying the initial instruction address of a new thread and a second operand. The microprocessor executes the fork instruction by allocating context for the new thread, copying the first operand to a program counter of the new thread context, copying the second operand to a register of the new thread context, and scheduling the new thread for execution. If no new thread context is free for allocation, the microprocessor raises an exception to the fork instruction. The fork instruction is efficient because it does not copy the parent thread general purpose registers to the new thread. The second operand is typically used as a pointer to a data structure in memory containing initial general purpose register set values for the new thread.