The International Socialist Organisation (ISO) was formed in 1977 by members of the International Socialists who were concerned at at the groups turn away from its advocacy of building a revolutionary party through the construction of a rank and file movement within the trade unions. Another dimension was added to the factional struggle in the IS as the oppositional members of the IS had become convinced of the positions of the International Socialists on the question of the class nature of the Stalinist states, which they held to be "state capitalist" and on the then current Portugese Revolution. The new ISO, led by Cal and Barbara Wilson, quickly affiliated with the British Socialist Workers Party (SWP/UK) and what was to become the International Socialist Tendency. They also began publishing a monthly newspaper (Socialist Worker).
In the early 1980s, the Wilsons were removed from the leadership of the ISO and replaced when Ahmed Shawki (a former SWP/UK member) moved to the United States. Under Shawki's leadership, the small ISO group grew by organizing on university campuses. Although he remained in the post split IS Joel Geier later joined Shawki and Sharon Smith in the leadership of the ISO. They began to spend less time discussing theoretical issues, and more time organising around individual issues.
While claiming to have between 800-1000 members, the ISO goes through a high rate of member turnover (i.e., people join and leave the group quickly). This is because they demand every member be a cadre (activist) — selling ISO papers, organizing and funding ISO events, demanding a high level of commitment and time. Internal debate within the organisation was also highly controlled by the leadership at the centre.
In 2000, the ISO picked up a large number of new members by working and supporting the presidential campaign of the Green Party's Ralph Nader. In 2001, after a battle between Shawki and the leaders of the SWP/UK, the ISO was expelled from the International Socialist Tendency.
A small group of ISO members broke off to form a new American section of the IST (this group calls itself Left Turn). (Note: Left Turn broke with the IST in 2003.) Subsequently, the ISO has begun to establish new international relations, especially with other groups, such as Socialist Alternative in Australia, who have been expelled or left the IST.
Besides publishing Socialist Worker, the ISO also publishes a magazine, the International Socialist Review.