Nicolas Panard was arrested on Tuesday at his home in Mulhouse in eastern France and taken to the nearby town of Montbeliard for questioning.
His former lover, Slim Fezzani, a 43-year-old Tunisian illegal immigrant who is serving a jail term for the murder of a gay insurance agent in 1995, is suspected of being his accomplice.
Investigators have always believed that the murder, described as being "extremely violent", was not committed by Fezzani alone. He was transferred from his jail near Paris to Montbeliard for questioning.
Panard worked in a string of cabarets in eastern France and in Switzerland and Germany, dressing up as Marilyn Monroe and French singers Mireille Mathieu and Dalida, until he retired in 1992.
The 18 murders of which he is suspected were committed between 1980 and 2000, but prosecutors said the current investigation concerned six killings which took place between 1983 and 2000. One was a woman and five were men, one of whom was handicapped. All had been beaten and stabbed, and one was killed with a crucifix.