Viacom President Mel Karmazin has resigned his post, according to the company.

Following the unexpected move, MTV Networks head Tom Freston and CBS chief Les Moonves have been appointed to each fill a Karmazin shoe with the titles of Co-President and Co-COO, effective immediately. In addition to MTV Networks, Freston will now be responsible for the operations of Showtime, BET, Paramount Pictures and Simon & Schuster. In addition to CBS, Moonves will now be responsible for Paramount TV, Infinity Broadcasting and Viacom Outdoor.

But wait, there’s more: The management shakeup, announced today, is part of a corporate succession plan that will see CEO Sumner Redstone step down as Viacom’s CEO some time in the next three years. The plan, developed by Redstone and the Viacom board’s Governance Committee, calls for Redstone to work with the board in the interim to identify his successor and candidates for other key posts.

“Obviously, Tom and Les are the two leading candidates to be the next CEO of Viacom,” Redstone said in a statement, noting that both execs had presided over “phenomenal growth stories” during their tenures.

Redstone and Karmazin were known to have a contentious relationship over the years, with recent reports saying new disagreement had broken out over what growth strategy to pursue. Karmazin’s contract is said to have an unusual clause allowing him to exit the company at will—reportedly with a large payout—if Redstone were to override him on matters of strategy.

“We very much regret Mel’s decision to resign and we wish him well,” Redstone said. “He has been instrumental in Viacom’s operating success since our merger with CBS and he leaves with an extraordinary track record of accomplishment.” Karmazin has agreed to remain as a consultant for two months to help Freston and Moonves with the transition.

Karmazin was named President/COO of Viacom in 2000 upon its purchase of CBS, which had previously bought Karmazin's Infinity—syndicator of Howard Stern, among other things. Said Karmazin of his departure, “After more than 20 years with the Company, for personal and professional reasons, I have decided to leave Viacom and pursue other challenges. Viacom is performing exceptionally well, with leadership positions in all of its businesses. The company’s very talented management team will ensure its continued success.”

Of his new role, Freston said: “I am excited about the opportunity to play a greater role at Viacom. After so many years at MTV and MTV Networks, Viacom is a company that I love, and one that I know intimately. I have had the privilege to play a key role in MTV Networks’ expansion and Viacom’s operational and financial growth since the beginning, and I relish this new challenge. I am looking forward to working with Sumner, Les and all my talented colleagues at Viacom who I have collaborated with for many years. With our strong vision, our vibrant culture and our incredible brands, we will further strengthen Viacom's position as the premier media company.”

Said Moonves, “It has been extremely gratifying to play a role in the CBS comeback over the past several years, working with a great team. I am excited to take on these new responsibilities at such an important moment in the evolution of Viacom, and I am thrilled that I will be working so closely with Sumner and Tom as we chart the future for this great Company. Viacom’s assets and management are the best in the business, and I know that working together we will take Viacom to new heights.”