Mr. Zeisler, one of the founders of the Guthrie Theater in Minneapolis and the longtime executive director of the Theatre Communications Group, an advocacy organization that publishes American Theatre magazine, was 81. The cause of death was heart failure.

"This truly marks the end of an era for us here at TCG and represents the loss of a seminal figure for our field," Ben Cameron, executive director of Theatre Communications Group, told Playbill.com. "TCG will always bear the imprint of Peter’s personality: indeed, the programs that are now critical for the field—the book program, the magazine, the fiscal research and surveys, the American Arts Alliance, and more—were founded under Peter’s remarkable leadership.  Even more, he was a man of uncompromising conscience—one who would speak openly and passionately about freedom of expression, diversity and artistic conscience from the viewpoint of one who had been blackballed in the industry, who had faced disciplinary action for daring to salute an officer of color on a Southern military base—in short, a man with a wealth of experiences.  He was a mentor to many leaders and theatres who literally would not have been in the field today without him.  He will be deeply, deeply missed."

Mr. Zeisler's life in resident theatre advocacy was preceded by work as a stage manager in the commercial world of Broadway, where he worked in the 1950s and '60s on such shows as The Sound of Music, Juno, A Hole in the Head, Candide (directed by the Tyrone Guthrie), Champagne Complex, All Summer Long and The Shrike.

But he grew weary of the limitations of the commercial arena, and with famed director Tyrone Guthrie sought out a city that would support a serious classical theatre. The 1963 founding of the Guthrie Theater in Minneapolis (decided upon after visits to seven cities around the country) was a major landmark on the national regional (or "resident," as some prefer) theatre movement.

According to Guthrie historical notes, the idea of the theatre began in 1959 during a series of conversations among Guthrie and two colleagues — Oliver Rea and Mr. Zeisler — who were disenchanted with Broadway. They wanted to create a theatre "with a resident acting company that would perform the classics in rotating repertory with the highest professional standards." According to the company's website, "The Guthrie became a prototype for an important new kind of theatre in contrast to the commercial environment of Broadway. There, the high costs associated with mounting a production increasingly mandated that shows must be immediately successful at high ticket prices. The Broadway atmosphere was conducive neither to producing the great works of literature, nor to cultivating the artists' talents, nor to nourishing the audience."

The collaborators "were drawn to Minneapolis/St. Paul because of its location in the heartland of America, the vitality of the cultural community, the presence of a large state university and many small colleges, and the enthusiasm shown by the Upper Midwest for the new theatre project."

The theatre opened with 22,000 season ticket holders and $300,000 in advance sales for its first May-September season of four plays. The Ford Foundation provided funds to offset operating losses for the first six years.

Hume Cronyn and Jessica Tandy were in the company in its first season.

Mr. Zeisler was managing director and producer from 1963 to 1970.

In 1982 the Guthrie received a special Tony Award for its contribution to the American theatre.

In 1995, Mr. Zeisler retired as executive director of Theatre Communications Group, a post he held since 1972. He continued to be a theatre consultant.

Mr. Zeisler also helped founding the American Shakespeare Festival in Stratford, CT.

He is survived by his wife, the actress Helen Harrelson, and two sons, Mark and Eric.

The family has requested that, in lieu of flowers, contributions be made in Mr. Zeisler's memory to TCG, for programs in support of theatre artists.  Contributions, payable to Theatre Communications Group, may be sent to TCG, 520 Eighth Avenue, 24th Floor, New York, NY 10018.