Abstract. A proper edge-coloring with the property that every cycle contains edges of at least three
distinct colors is called an acyclic edge-coloring. The acyclic chromatic index of a graph G, denoted
χ 0a
ðGÞ, is the minimum k such that G admits an acyclic edge-coloring with k colors. We conjecture that if
G is planar and ΔðGÞ is large enough, then χ 0a
ðGÞ ¼ ΔðGÞ. We settle this conjecture for planar graphs with
girth at least 5. We also show that χ 0a
ðGÞ ≤ ΔðGÞ þ 12 for all planar G, which improves a previous result by
Fiedorowicz, Haluszczak, and Narayan [Inform. Process. Lett., 108 (2008), pp. 412–417].