HALBERSTADT, MORDECAI (also known as Mordecai of Duesseldorf; d. 1770), rabbi and grammarian. Born in the town of Halberstadt at the beginning of the 18th century, Mordecai studied under Abraham b. Judah Berlin, the local rabbi, and Ẓevi Hirsch Ashkenazi, the head of its yeshivah. He proceeded to Frankfurt in 1730 where he studied under Jacob ha-Kohen, author of Shav Ya'akov, whose rulings and responsa he quotes in his Ma'amar Mordekhai (nos. 10, 69, 70, et al.). He taught at the Halberstadt yeshivah and, on the recommendation of Jacob ha-Kohen, was appointed av bet din of Griesheim near Frankfurt (Ma'amar Mordekhai, nos. 2, 8, 14). He later served as rabbi of Darmstadt, and then at Duesseldorf (no. 23), where he remained until the end of his life. Requested by Samuel Heilmann of Metz and Joshua *Falk of Frankfurt to join in the ban against Jonathan *Eybeschuetz and to give his opinion about the amulets, Halberstadt was reluctant to attack Eybeschuetz personally and instead recommended that they content themselves with adverse criticism of the activities of the circles close to Shabbateanism. He was the author of the responsa, Ma'amar Mordekhai (Bruenn, 1790). Responsum no. 30 deals with the case of an animal in whose stomach was found a needle adhering to the midriff. The scholars of the Rhineland regarded such an animal kasher on the basis of responsa by Ephraim Solomon *Luntschitz and Isaiah *Horowitz (ibid., 41b). Halberstadt proved with profound acumen that these alleged responsa were forgeries by the Bonn informer, Krauss, "who forged and testified falsely in the names of those great scholars." Leḥem Eden, a pamphlet containing the glosses of Halberstadt's son, menahem mendel halberstadt, is appended to the book. Mordecai Halberstadt also compiled a work on grammar that has remained in manuscript. His grandson, who published the Ma'amar Mordekhai, refers to him as "Mordecai Balshan [the 'linguist'], because of his profound knowledge of the holy tongue and Hebrew grammar."
B.H. Auerbach, Geschichte der israelitischen Gemeinde Halberstadt (1866), 74–76, no. 11; idem (Ẓevi Binyamin), Berit Avraham (1860), 24f.; P. Frankl, in: Nachlath Zvi, 8 (1937), 79.