OSTROW MAZOWIECKA (Pol. Ostrów Mazowiecka; Russ. Ostrov Lomzinsky), town in the province of Warszawa, N.E. Central Poland. The intolerant attitude of the authorities of Masovia prevented the settlement of Jews for several centuries, and it was only during the 8th century that Jews succeeded in establishing themselves there permanently. In 1765 there were 68 Jews (20 families) paying the poll tax and owning 15 houses in the town, and another 45 Jews in six surrounding villages. Seven heads of families earned their livelihood from crafts; the remainder engaged in retail trade or held leases. In 1789 a Polish tribunal issued a restriction against Jewish settlement in the town, which remained in force until 1862. Jews who succeeded in settling in Ostrow Mazowiecka came mostly from central Poland and Lithuania, developing a special Yiddish dialect which combined the Yiddish language features of both areas. In spite of prohibitions there were 382 Jews living in Ostrow Mazowiecka in 1808 (34% of the total population). In 1827 they numbered 809 (39%). Jews engaged essentially in retail trade, peddling, haulage, and tailoring. In 1857 the community numbered 2,412 (61% of the population). A few wealthy families traded in wood and grain, and worked flour and saw mills. From 1850 the community supported a yeshivah. During the second half of the 19th century (somewhat later than in most other places) a dispute broke out between the Ḥasidim and the *Mitnaggedim in the community. Rabbis of the two factions officiated alternately, notably David Solomon Margolioth, Judah Leib *Gordon, and the ẓaddik Gershon Ḥanokh of Radzyn. The majority of the local Ḥasidim belonged to the Gur (*Gora Kalwaria) and *Warka dynasties. In 1897 the Jewish community numbered 5,910 (60% of the population). Although at the beginning of the 20th century religious and secular Jewish educational institutions were established, it was not until the end of World War I that the community's institutions were organized to their fullest extent. In 1921, 6,812 Jews (51% of the total) made up the community's population. In 1934 the Jews of Komorowo were incorporated into the community of Ostrow Mazowiecka, and the yeshivah Beit Yosef was transferred to the town in 1922.
R. Mahler, Yidn in Amolikn Poyln in Likht fun Tsifern (1958), index; B. Wasiutyński, Ludność żydowska w Polsce w wiekach XIX i XX (1930), 36, 66, 72, 77, 79; I. Schiper (ed.), Dzieje handlu żydowskiego na ziemiach polskich (1937), index; Sefer ha-Zikkaron li-Kehillat Ostrów Mazowieck (Heb. and Yid., 1960); Ostrow Mazowieck (1966), a memorial book publ. in Heb.