HARRISBURG, Pa., Sept. 26 - Intelligent design is not science, has no support from any major American scientific organization and does not belong in a public school science classroom, a prominent biologist testified on the opening day of the nation's first legal battle over whether it is permissible to teach the fledgling "design" theory as an alternative to evolution.

"To my knowledge, every single scientific society that has taken a position on this issue has taken a position against intelligent design and in favor of evolution," said the biologist, Kenneth R. Miller, a professor at Brown University and the co-author of the widely used high school textbook "Biology."

Eleven parents in the small town of Dover, just south of here, are suing their school board for introducing intelligent design in the ninth-grade biology curriculum. The parents accuse the board of injecting religious creationism into science classes in the guise of intelligent design. Professor Miller, their main expert witness, was the only person to take the stand on Monday.

Intelligent design is the idea that living organisms are so complex that the best explanation is that some kind of higher intelligence designed them. The notion has gained a foothold in some states and school districts as an attractive alternative to evolution, but is shunned by most mainstream scientists.

In a sign of how important this trial is to the adversaries in the intelligent design debate, they came from across the country to hear the opening arguments and to present their case to the cameras waiting outside. The two sides agree that no matter how Judge John E. Jones III decides the case in Federal District Court here, it will probably make its way to the Supreme Court.

Casey Luskin, a program officer at a group that advocates intelligent design, the Discovery Institute, said in an interview outside the courtroom: "No one is pretending that intelligent design is a majority position. What we're rebutting is their claim that there's no controversy among scientists."

The school board members, represented by a nonprofit Christian law firm based in Michigan, are taking the stance that students should have access to a variety of scientific theories.

"This case is about free inquiry and education, not about a religious agenda," Patrick Gillen, a lawyer for the board said in his opening statement.

The board president, Sheila Harkins, said in an interview during a break, "The whole thought behind it was to encourage critical thinking."

It was "not true at all," Ms. Harkins said, that board members were motivated by their religious beliefs.

The front rows of the courtroom were filled on one side with members of the Dover school board, the defendants, and on the other, the Dover residents who filed suit. On both sides of the aisle the mood was grim, and there was barely a look or a handshake exchanged across it.

The plaintiffs are trying to show that intelligent design is just "the 21st-century version of creationism," as a lawyer for the plaintiffs, Eric Rothschild, put it in his opening argument.

Mr. Rothschild said that the board's own documents would show that the board members had initially discussed teaching "creationism" - one former member said he wanted the class time evenly split between creationism and evolution - and that they substituted the words "intelligent design" only when they were made aware by lawyers of the constitutional problems involved.

The board ultimately settled for directing that a four-paragraph statement be read to the students at the opening of the semester's biology class. It says, in part: "Because Darwin's theory is a theory, it continues to be tested as new evidence is discovered. The theory is not a fact. Gaps in the theory exist for which there is no evidence."

The statement says that "intelligent design is an explanation of the origin of life that differs from Darwin's view," and it advises students that a textbook that teaches intelligent design, "Of Pandas and People," is available in the school library.

In his testimony, Professor Miller called the Pandas textbook "inaccurate and downright false in every section." The board's statement "undermines sound science education" by conveying to students that only evolution merits such skepticism, he said.

Professor Miller projected slides that he said contradicted the core of design theory: that organisms are irreducibly complex. He also denigrated intelligent design as "a negative argument against evolution," in which there is no "positive argument" to test whether an intelligent designer actually exists. If the theory is not testable, he said, it is not science.

Randall Wenger, a lawyer for the Foundation for Thought and Ethics, the intelligent design advocacy group that produces the Pandas textbook, said, "If they decide that intelligent design is just a remake of creationism, that horribly undermines" both the Pandas textbook and "the motivation for scientists to study intelligent design."

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