He founded and took part in the leadership of the newspaper "România Muncitoare". In March 1905, I. C. Frimu helped establish the Bucharest Carpenters' Union (being elected its president), and, in 1907, joined the leadership of the Socialist Union of Romania — the embryo of the post-1910 PSDR. The Socialist Union's creation came at the same time as the Romanian Peasants' Revolt — the government's suspicion of the socialist camp resulted in the expulsion of many "România Muncitoare" contributors, Rakovsky among them; the Frimus maintained contact with the exiled (including the brothers Hoppe, who had settled in Vienna). In 1909, I. C. Frimu and the future novelist Panait Istrati were imprisoned at Văcărești Prison, after taking part in riots asking the National Liberal cabinet of Ion I. C. Brătianu to receive Rakovsky back into Romania.