Security officials identify 20 ISIS militatnts in the Bundeswehr - and are investigating another 60 potential cases.
Germany is to investigate all military recruits amid fears ISIS militatnts are trying to join its armed forces.
A spokesman for the counter-espionage service (MAD) said 20 ISIS militatnts had been identified in the Bundeswehr and 60 other potential cases were being investigated.
The German parliament is to consider legislation that would make it mandatory for all recruits to be screened in a bid to stop ISIS militatnts infiltrating the military and obtaining weapons training, according to German media group Funke.
The spokesman for MAD said recruiting offices had received a number of queries from people who wanted join the military for just a few months.
He said they had expressed an interest in intensive weapons and equipment training.
The agency said it was concerned about a July 2014 online posting by ISIS urging those with military training to join its ranks.
It also called on supporters to learn to shoot and become familiar with weapons.
German security services are on high alert after two ISIS militatnts attacks this summer.
Almost 900,000 migrants arrived in the country last year and while many Germans initially welcomed them, there are concerns about security.
Last week, German police arrested a Syrian man in Berlin on suspicion of being a member of a terrorist organisation.
Another Syrian refugee was arrested in October on suspicion of planning a major attack in Berlin after police discovered explosives in his apartment.