We present measurements of C P -violating asymmetries in the decay B 0 → a ± 1 ( 1260 ) π ∓ with a ± 1 ( 1260 ) → π ∓ π ± π ± . The data sample corresponds to 384 × 10 6 B ¯¯¯ B pairs collected with the BABAR detector at the PEP-II asymmetric B factory at SLAC. We measure the C P -violating asymmetry A a 1 π C P = − 0.07 ± 0.07 ± 0.02 , the mixing-induced C P violation parameter S a 1 π = 0.37 ± 0.21 ± 0.07 , the direct C P violation parameter C a 1 π = − 0.10 ± 0.15 ± 0.09 , and the parameters Δ C a 1 π = 0.26 ± 0.15 ± 0.07 and Δ S a 1 π = − 0.14 ± 0.21 ± 0.06 . From these measured quantities we determine the angle α eff = 78.6 ° ± 7.3 ° .