We study the twisted Alexander polynomial ∆ K,ρ of a knot K associated to a non-abelian representation ρ of the knot group into SL 2 (C). It is known for every knot K that if K is fibered, then for every non-abelian representation, ∆ K,ρ is monic and has degree 4g(K) − 2 where g(K) is the genus of K. Kim and Morifuji recently proved the converse for 2-bridge knots. In fact they proved a stronger result: if a 2-bridge knot K is non-fibered, then all but finitely many non-abelian representations on some component have ∆ K,ρ non-monic and degree 4g(K)−2. In this paper, we consider two special families of non-fibered 2-bridge knots including twist knots. For these families, we calculate the number of non-abelian representations where ∆ K,ρ is monic and calculate the number of non-abelian representations where the degree of ∆ K,ρ is less than 4g(K) − 2.2010 Mathematics Subject Classification. 57M27.