Colombia says the man considered to be the top military chief of the FARC rebel group has been killed in combat.
Mono Jojoy is reported to have been killed in an air strike in the region of La Macarena.
Observers say the death is a severe strike against FARC, which lost two other commanders in 2008.
Colombian media said the veteran guerrilla had more than 60 arrest warrants issued against him.
The government had also been offering a large reward for his capture.
Jojoy was accused of being responsible for holding hostages including politicians, police officers and soldiers.