Filemon Lagman, Filipino theoretician and activist (b. 1953)
Filemon Castelar Lagman (March 17, 1953 – February 6, 2001), popularly known as Ka Popoy, was a revolutionary socialist and workers' leader in the Philippines. He shares the ideology of Marxism-Leninism. He split with the Communist Party of the Philippines in 1991 to form Bukluran ng Manggagawang Pilipino (BMP) and the multi-sectoral group Sanlakas.

From the split, he led the formation of the Partido ng Manggagawang Pilipino (Filipino Workers' Party), an underground revolutionary socialist party, which, after his death, merged with the Sosyalistang Partido ng Paggawa (Socialist Party of Labor) and the Partido para sa Proletaryong Demokrasya (Party for Proletarian Democracy).

During the First Quarter Storm, he was a member of Samahang Demokratiko ng Kabataan (Democratic Association of the Youth) in the 1970s. After only a year in college at the University of the Philippines, he decided to go underground and do full-time organizing work in the factories and urban poor communities in the northern sector of Metro Manila. Ka Popoy was elected Secretary of the Manila-Rizal Regional Party Committee of the CPP in the mid-70's and spearheaded the broad formation which challenged the Marcos dictatorship in 1978 Batasan Pambansa elections. The Central Committee of the CPP admonished Ka Popoy and the whole regional committee for advocating participation in 1978 Batasan Pambansa elections because it ran counter to the CC call to head to the countryside to wage armed struggle against the dictatorship. Ka Popoy was only to return at the helm of the Manila-Rizal Regional Party Committee after the EDSA uprising of 1986. In spite of the differences with the central leadership, Ka Popoy strived harder to strengthen revolutionary work in the capital city.

Ka Popoy is also known to be the only Party leader that during the struggles with the CPP that put forward the most comprehensive and in-depth critique against the basic Party documents of CPP-NPA which are now popularly known as Counter-Thesis 1 (PSR: A Semi-feudal Alibi for Protracted War, PPDR: Class Line vs. Mass Line and PPW: A New-Type Revolution of the Wrong Type) and Counter-Thesis 2 (On the Reorientation of the Party Work and the Reorganization of the Party Machinery).

He was shot dead in Bahay ng Alumni, University of the Philippines Diliman in Quezon City on February 8, 2001. His assassination is speculated to have been carried out by a pro-Estrada faction of the military, aiming to destabilize the newly formed Arroyo government. Further investigation by the police revealed that the assassins and the culprits may have come from the Revolutionary Proletarian Army-Alex Boncayao Brigade, another "rejectionist" faction of the CPP. The perpetrators have not yet been apprehended as of 2008. On July 2007, the Quezon City Prosecutor’s Office decided to drop the case on eight suspected communist assassins since the witnesses were unable to attend the preliminary investigations.