A proper vertex-colouring of a graph is acidic if there are no 2-coloured cycles. It is known that every planar graph is acyclically 5-colourable. and that there are planar graphs with acyclic chromatic number chi (a) = 5 and girth g = 4. It is proved here that a planar graph satisfies chi (a) less than or equal to 4 if g greater than or equal to 5 and chi less than or equal to 3 if g greater than or equal to 7.