(pseudonym of Maria Koszutska). Born Feb. 2, 1876, in the village of Głowczyn, near Kalisz; died July 9, 1939. Leader of the Polish workers’ movement.

A teacher by profession, Kostrzewa joined the Polish Socialist Party (PSP) in 1902 and participated in the Revolution of 1905–07 in Poland. She helped organize the left wing of the PSP in 1906 and was a member of its central operational committee from 1906 to 1918. She was arrested and exiled many times for her revolutionary activity. Kostrzewa actively supported the merger, according to Marxist principles, of the left wing of the PSP and the Social Democratic Party of the Kingdom of Poland and Lithuania to form the Communist Party of Poland. She was a member of the Central Committee and of the Politburo of the Central Committee of the Communist Party of Poland from 1918 to 1930 (with interruptions), and played a significant role in working out the party’s program on the agrarian and national questions. She attended the Fourth, Fifth, and Sixth Congresses of the Comintern. Kostrzewa lived in the USSR from 1930 until her death.