Intelligent design is the belief that living organisms are so complex that the best explanation is that they were created by an intelligent force of some kind. At issue in the lawsuit is whether the concept's introduction into biology class is an abridgment of the separation between church and state.

The board voted last year to require that students in ninth-grade biology listen to a four-paragraph statement saying that there are gaps and problems with the theory of evolution and that intelligent design is among the alternatives worth considering. The statement said that among the resources available in the school library for further study is an intelligent-design textbook, "Of Pandas and People."

In two days on the stand, Professor Behe has insisted that intelligent design is not the same as creationism, which supports the biblical view that God created the earth and its creatures fully formed. The Supreme Court has ruled that creationism is a religious belief and cannot be taught in public school.

The cross-examination of Professor Behe on Tuesday made it clear that intelligent-design proponents do not necessarily share the same definition of their own theory. Eric Rothschild, a lawyer representing the parents suing the school board, projected an excerpt from the "Pandas" textbook that said:

"Intelligent design means that various forms of life began abruptly through an intelligent agency with their distinctive features already intact, fish with fins and scales, birds with feathers, beaks and wings, etc."

In that definition, Mr. Rothschild asked, couldn't the words "intelligent design" be replaced by "creationism" and still make sense? Professor Behe responded that that excerpt from the textbook was "somewhat problematic," and that it was not consistent with his definition of intelligent design.