The article presents Janusz Molka, one of the most dangerous secret collaborators of the Security Service (SB) in Tri-City, and reveals the full context of his registration by the 2 rd Department of KWMO (Voivodeship Command of Citizens’ Militia, or communist-era police) in Gdańsk. J. Molka was noticed by the SB already as a high school student. He was then active in youth circles associated with academic chaplaincy, painting “hostile” slogans on walls. In that time he met Arkadiusz Rybicki and Aleksander Hall. After August 1980, he actively joined in creating the structures of “Solidarity” trade union in Gdańsk. After the imposition of martial law, he handled the distribution of clandestine publications. In February 1982, the 3 rd Department of KWMO in Gdańsk launched an investigation code-named “Krokus”, due to the suspected use of the Tri-City Highland Club, of which Molka was a member, for illegal activities. SB functionaries began observing Molka. On 13 th May 1983 he was detained and recruited as a secret collaborator of SB, under the pseudonym “Majewski”. Due to secrecy needs he later changed his pseudonym to “C-5”, again to “Romkowski”. He was used to infiltrate opposition structures in Tri-City. Until October 1984 he delivered over 80 written reports. His performance was cited as outstanding exampole of cooperation. As a recognition of his merits, the SB command expressed consent to his formal hiring on a secret position (December 1984). From then on, he used the pseudonym “Nowak”. He maintained contacts with the underground authorities of “Solidarity”, the Independent Students’ Association (NZS) of Gdańsk University, he reached the persons exercising control over the transportation routes of clandestine literature. His work was very much appreciated. In 1989 he was transferred to the 3 rd Department of MSW (Ministry of Internal Affairs) in Warsaw. In May 1990 he left the Security Service, without being subjected to the procedure of verification. Checking out Janusz Molka in IPN’s (Institute of National Remembrance’s) auxiliary files, does not confirm his collaboration with the SB. Molka’s registration card features the number of registration 47075. The registration book at that number states the category OZ (protected person). However, another registration number appears in Janusz Molka’s personal files (officer’s files) – 47080. At that number, the registration book features notes about the TW (secret collaborator) pseudonym “Majewski”. But, that number is written on the registration card of Katarzyna A. An analysis of archives enables to assert that the mistake was committed by a functionary of the 3 rd Department of KWMO in Gdańsk, on filling the registration cards in the beginning of the registration process of Janusz Molka.