I recently completed a tour of Norwegian universities, where I spoke about international law as applied to the Israeli-Palestinian conflict. But the tour nearly never happened.

Its sponsor, a Norwegian pro-Israel group, offered to have me lecture without any charge to the three major universities. Norwegian universities generally jump at any opportunity to invite lecturers from elsewhere. When my Harvard colleague Stephen Walt, co-author of “The Israel Lobby,” came to Norway, he was immediately invited to present a lecture at the Norwegian University of Science and Technology in Trondheim. Likewise with Ilan Pappe, a demonizer of Israel who teaches at Oxford.

My hosts expected, therefore, that their offer to have me present a different academic perspective on the Israeli-Palestinian conflict would be eagerly accepted. I have written half a dozen books on the subject presenting a centrist view in support of the two-state solution. But the universities refused.

The dean of the law faculty at Bergen University said he would be “honored” to have me present a lecture “on the O.J. Simpson case,” as long as I was willing to promise not to mention Israel. An administrator at the Trondheim school said that Israel was too “controversial.”

The University of Oslo simply said “no” without offering an excuse. That led one journalist to wonder whether the Norwegian universities believe that I am “not entirely house-trained.”

Only once before have I been prevented from lecturing at universities in a country. The other country was Apartheid South Africa.

Despite the faculties’ refusals to invite me, I delivered three lectures to packed auditoriums at the invitation of student groups. I received sustained applause both before and after the talks.

It was then that I realized why all this happened. At all of the Norwegian universities, there have been efforts to enact academic and cultural boycotts of Jewish Israeli academics. This boycott is directed against Israel’s “occupation” of Palestinian land—but the occupation that the boycott supporters have in mind is not of the West Bank but rather of Israel itself. Here is the first line of their petition: “Since 1948 the state of Israel has occupied Palestinian land . . .”

The administrations of the universities have refused to go along with this form of collective punishment of all Israeli academics, so the formal demand for a boycott failed. But in practice it exists. Jewish pro-Israel speakers are subject to a de facto boycott.

The first boycott signatory was Trond Adresen, a professor at Trondheim. About Jews, he has written: “There is something immensely self-satisfied and self-centered at the tribal mentality that is so prevalent among Jews. . . . [They] as a whole, are characterized by this mentality. . . . It is no less legitimate to say such a thing about Jews in 2008-2009 than it was to make the same point about the Germans around 1938.”

This line of talk—directed at Jews, not Israel—is apparently acceptable among many in Norway’s elite. Consider former Prime Minister Kare Willock’s reaction to President Obama’s selection of Rahm Emanuel as his first chief of staff: “It does not look too promising, he has chosen a chief of staff who is Jewish.” Mr. Willock didn’t know anything about Mr. Emanuel’s views—he based his criticism on the sole fact that Mr. Emanuel is a Jew. Perhaps unsurprisingly, fewer than 1,000 Jews live in Norway today.

The country’s foreign minister recently wrote an article justifying his contacts with Hamas. He said that the essential philosophy of Norway is “dialogue.” That dialogue, it turns out, is one-sided. Hamas and its supporters are invited into the dialogue, but supporters of Israel are excluded by an implicit, yet very real, boycott against pro-Israel views.

Mr. Dershowitz is a law professor at Harvard. His latest novel is “The Trials of Zion” (Grand Central Publishing, 2010).