We prove that the acyclic chromatic index a′ (G) ≤ 6 Δ for all graphs with girth at least 9. We extend the same method to obtain a bound of 4.52 Δ with the girth requirement g ≥ 220. We also obtain a relationship between g and a′ (G). © 2007 Elsevier B.V. All rights reserved.