"The Walking Dead" showrunner's third season premieres to 10.9 million viewers and a 5.8 rating in the 18-to-49 demo, bigger than any entertainment series this fall on broadcast or cable.

AMC has renewed The Walking Dead for a fourth season -- without showrunner Glen Mazzara, who has parted ways with cable's top series.

"Both parties acknowledge that there is a difference of opinion about where the show should go moving forward and conclude that it is best to part ways," AMC said in a statement. "This decision is amicable, and Glen will remain on for postproduction on season 3B as showrunner and executive producer. (Filming on season three already has concluded, with the second half of episodes set to return to AMC in February.)

A replacement has yet to be announced, but sources tell The Holywood Reporter that the series might elevate a current member of the show's writing staff.

Mazzara's contributions to the zombie drama have been widely hailed by critics, with the series landing on several best of 2012 lists (including that of THR's Tim Goodman) and being praised for its turnaround in pacing -- THR deemed the series a 2012 Rule Breaker for its willingness to take big swings, including killing off major characters midseason.

Mazzara, who as a freelancer penned a season-one episode, was brought in as an exec producer under Frank Darabont ahead of season two. He ultimately was tapped to replace Darabont, who helped shepherd the series to the screen and was fired after refuting budget cuts to the drama.

"My time as showrunner on The Walking Dead has been an amazing experience, but after I finish season three, it’s time to move on," Mazzara said in a statement. "I have told the stories I wanted to tell and connected with our fans on a level that I never imagined. It doesn’t get much better than that. Thank you to everyone who has been a part of this journey."

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Mazzara becomes the latest in a line of showrunner issues at AMC, which most recently renewed period drama Hell on Wheels for a third season after creators/showrunners Joe and Tony Gayton departed the Western and John Shiban was poised to return. However, he recently exited the series with John Wirth tapped as its third showrunner in as many seasons. In addition, Rubicon creator-showrunner Jason Horwitch exited the series in 2010. The cable network also went through protracted contract negotiations with Breaking Bad's Vince Gilligan and Mad Men's Matthew Weiner, which are also said to have turned contentious after rumored budget cuts.

Walking Dead, based on the comics created by Robert Kirkman, opened its third season in October right where it left off: shattering its own basic-cable ratings records. The series, starring Andrew Lincoln as the leader of a band of survivors navigating the postapocalyptic world with threats in the form of the living and the dead, now boasts the biggest telecast for any drama series in basic-cable history with 10.9 million total viewers tuning in to the Oct. 14 season premiere. It ranks as the No. 1 show of the fall in the demo, beating everything on broadcast. The season three opener continued the series' trend of besting its own records, with its season 2 premiere, midseason premiere and finale all setting bests in the key demographics.

In the demos, the opener scored 7.3 million in 18-49 and 6.1 million in 25-54, all series bests. Factoring in repeats, the series collected a whopping 15.2 million total viewers during the course of the night. Its second episode slid to 9.5 million total viewers and 6.5 million in 18-49, topping all nonsports programming for the night in the demo, rebounding in week three to 10.5 million total viewers and a 5.4 in the demo. In its first half, which wrapped in November, the series is averaging 10.1 million total viewers per week.

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"I am in full support of both AMC and Glen Mazzara in the decision they have come to and believe the parties came to this decision in the best interest of the future of the show," Kirkman said in a statement. "I thank Glen for his hard work and appreciate his many contributions to The Walking Dead and look forward to working with him as we complete postproduction on season three. I am also excited to begin work on another spectacular season of this show that I know means so much to so many people. This show has always been the result of a wide range of extremely talented men and women working tirelessly to produce their best work collectively. I believe the future is bright for The Walking Dead. Thank you to the fans for your continued support."

Added EP Gale Anne Hurd: “I am appreciative and grateful to Glen for his hard work on The Walking Dead. I am supportive of AMC and Glen's decision and know that the series is in great hands with one of the most talented and dedicated casts and crews in the business. I look forward to the show's continued success."



STORY: 'The Walking Dead': Glen Mazzara on Taking Over for Frank Darabont and His Fears as the Drama's New Showrunner

Season three also marks Walking Dead's largest with 16 episodes, up from six and 13 in seasons one and two, respectively. The season again was split in two, with eight episodes having aired in 2012 and the show set to return from its midseason hiatus Feb. 10.

The renewal was more of a when and not if as AMC opened a contest in late September offering blood donors a chance to win a trip to the set of The Walking Dead season four -- ahead of its October return.

Meanwhile, AMC is growing its commitment to The Talking Dead, with the Chris Hardwicke-hosted talk show expanding to an hour for the second half of its third season and moving up an hour to immediately follow the zombie drama at 10 p.m.

Email: Lesley.Goldberg@thr.com; Twitter: @Snoodit