Akinjide Oshuntokun, head of the history department at the University of Lagos, has been held without charge for eight weeks by Nigeria's State Security Service.
Professor Oshuntokun was intercepted by security service officers when he returned to Lagos airport from Germany, where he was collecting materials and data for a lecture on Nigeria's environmental crisis.
The officers told him to report for a "chat" at their Lagos headquarters. Despite efforts by the local branch of the Academic Staff Union of Universities (ASUU) to find out why he was arrested, there has been no official statement.
Professor Oshuntokun, until recently Nigeria's ambassador to Germany, confided to friends that top officials in General Sani Abacha's military regime were unhappy at his close links with the Friedrich Ebert Foundation, the German-based pro-environmental organisation of which he has been a trustee for some time and which sponsors his lectures.
"His environmental campaign, which has indicted the military government as a major violator of the environment through inadequate attention to problems of environmental degradation, has not gone down well with government officials," said one of his university colleagues who, for security reasons, asked not to be named.
While he was ambassador to Germany, Professor Oshuntokun criticised the execution of environmentalist and Ogoni rights campaigner Ken Saro-Wiwa. He was recalled from his post for welcoming Nigeria's Nobel Laureate, Wole Soyinka, during a visit the writer paid to Germany.
Since Professor Oshuntokun returned to the university, his academic activities have been a concern to the ruling junta. In a lecture on the role of the military since independence, he said, among other things: "The military is responsible for the political and economic woes plaguing the nation."
He claimed Yoruba civil servants were being selectively retired and even victimised for their opposition to the annulment of the 1993 presidential election, the winner of which, Moshood Abiola, himself a Yoruba, is still in detention.
On another occasion, he criticised Tom Ikimi, the minister of foreign affairs, for his response to the decision by the Commonwealth to suspend Nigeria because of its blatant violation of human rights.
"He knew that he was being trailed by the security agents," a colleague said. "As a scholar who believes in truth as an instrument of freedom, he was determined to pursue his intellectual pursuits in spite of obvious danger to his life."
Nigeria's Civil Liberties Organisation included Professor Oshun-tokun's name on the list of 60 prisoners of conscience presented to Pope John Paul II during his visit, which ended this week with a renewed appeal for restoration of human rights. The CLO announc- ed it would soon launch an international campaign for Professor Oshuntokun's release.