According to Marvel.com Michael Douglas will be playing the part of Hank Pym, while Paul Rudd will be the part of Scott Lang.

Marvel Studios President Kevin Feige had this to say:

“With Hank Pym’s rich history in the Marvel Universe, we knew we needed an actor capable of bringing the weight and stature to the role that the character deserves. We felt incredibly relieved when Michael Douglas agreed to step into the part with the charm and fortitude he brings to every character he inhabits, and couldn’t be more excited to see what he will do to bring Hank Pym to life.”

Edgar Wright  told Superherohype in an interview some time ago.

“The idea that we have for the adaptation is to actually involve both… Henry Pym and Scott Lang, So you actually do a prologue where you see Pym as Ant-Man in action in the 60’s, in sort of ‘Tales to Astonish’ mode basically, and then the contemporary, sort of flash-forward, is Scott Lang’s story, and how he comes to acquire the suit, how he crosses paths with Henry Pym, and then, in an interesting sort of Machiavellian way, teans up with him.”

Ant-Man is scheduled to hit theaters July 31st 2015.