He was arrested in France for possession of illegal firearms and served 2 years in a French prison. In the prison he presumably got insight into terrorist activities like bomb making, assassination and kidnapping. In August of 1970, he and other terrorists from Quebec traveled to Jordan to train with Palestinian commandos. At the same time, members of their group in Montreal announced they would begin a policy of "selective assassination."
Part of the terrorist group that caused the 1970 October Crisis, Yves Langlois was a member of the Liberation Cell of the FLQ that kidnapped James Cross, the British Trade Commissioner to Canada. Langlois and his terrorist group held James Cross hostage, taking his photo and sending it to police with a list of demands that included money and the release of other convicted terrorists. The kidnappers advise authorities that Cross will be executed and further threats to Cross' life were delivered to several radio stations along with the terrorists demands.
After he and the members of his terrorist cell were found, they negotiated their freedom and were exiled to Cuba.
On June 26, 1974, a report from the Canadian embassy in Paris, France indicated that Yves Langlois was living in Paris.
Yves Langlois returned to Quebec from exile in Paris in June of 1982. He was arrested and charged for his crimes and on September 27, 1982 he was sentenced to two years in prison less one day for his part in the kidnapping of James Cross. On July 19, 1983 -- less than one year later -- he was released on parole.