As many as several hundred teaching and research assistants at Columbia University canceled classes, refused to grade papers, and walked out of laboratories yesterday, the start of a planned one-week strike aimed at putting pressure on the administration to recognize their union. The strike was launched in coordination with a similar one at Yale University.

Columbia officials showed no signs of backing down from their refusal to recognize the graduate students’ union and said they expected fewer classroom disruptions than last year, when graduate students went on strike during the last four weeks of the spring semester. Yale officials likewise said they expected only limited disruptions.

“It’s very significant that we are out for a second time,” a doctoral student in philosophy at Columbia, Dehlia Hannah, said. A teaching assistant in two courses this semester, Ms. Hannah was one of the students picketing in front of the university gates at Broadway yesterday. She later attended a City Hall hearing about the strike.

Ms. Hannah said some professors in her department and others have agreed to hold their classes at off-campus locations, such as Riverside Church and the Cathedral of St. John the Divine on Amsterdam Avenue.

In the year between the two strikes at Columbia, union organizers suffered a heavy blow when the National Labor Relations Board ruled in July, on a 3-2 vote, that the teaching and research assistants are primarily students and not employees, and that it was likely that recognizing their union would hurt the “educational process.”

This decision by the board, which currently has a Republican majority, reversed a position it took in 2000 during the Clinton administration when it ruled in favor of graduate students’ unions at private universities. The 2004 decision also left unresolved the status of the graduate students’ union at New York University, whose contract expires this summer.

Columbia officials share the board’s current view. The provost, Alan Brinkley, in a statement to the university community, said it was Columbia’s position that “graduate teaching fellows and research assistants are students, not employees.”

It was not clear how many teaching and research assistants are participating in the walkout, as Local 2110 of the United Auto Workers union, which represents the graduate students, declined to give out specific numbers regarding the strike.

The president of UAW Local 2110, Maida Rosenstein, said a “majority” of the “more than 1,000” Columbia teaching and research assistants who are members of the union are taking part in the strike. Columbia has about 1,800 doctoral students who work as teaching and research assistants, according to figures provided by the university.

A Columbia freshman, Naftali Weinberger, 19, said his teaching assistant canceled classes this week in a “university writing” course that is mandatory for undergraduates. “In some way, having the extra time is helpful,” Mr. Weinberger said.

Union organizers and graduate students on strike have argued that teaching and research assistants take on key responsibilities for meager pay. By recognizing the union, the university would encourage the students to focus more on their teaching obligations, they say.

“They can focus on the teaching, and they don’t have to get second jobs,” Ms. Rosenstein said. She also said union recognition could lead to more comprehensive health-plan benefits.

Columbia offers the assistants a minimum annual stipend of $18,000 – expected to increase by $1,000 in the next academic year – and some research assistants receive up to $25,000. The assistants receive the same basic health coverage extended to undergraduates and are eligible for student housing with rents cheaper than comparable housing at Morningside Heights. The teaching assistants were not docked for last year’s job action.

While not achieving victory, graduate students urging the university to recognize their union have seen a sharp increase in the minimum stipend, which has risen by $6,000 since 2000. Columbia has sought to match the stipends offered at NYU, the first private university to recognize a graduate students’ union. Columbia graduate students initially voted to form a union three years ago in a secret ballot, submitting results to the labor board for recognition. The board impounded the votes after Columbia appealed the election and eventually defeated the move.