We present a fault collapsing procedure for transition faults based on fault dominance relations. The effectiveness of the procedure is enhanced by introducing what we call extended transition faults. A standard transition fault involves a single line and a transition. A transition fault from the value to the value a' on a line g is represented as g = a rarr g = a'. An extended transition fault involves two different lines with arbitrary values, and it is represented as g1 = a1 rarr g2 = a' 2. We demonstrate the importance of extended transition faults in fault collapsing, and describe two fault collapsing procedures. We consider the effects of fault collapsing on test generation