Mladá Boleslav (məläˈdä bôˈlĕsläf) [key], Ger. Jungbunzlau, city (1991 pop. 44,459), N central Czech Republic, in Bohemia. The city is an industrial center that manufactures automobiles, tractors, and agricultural machinery. Founded in the 10th cent., it became a center of the Bohemian Brethren (see Moravian Church) in the 15th to 16th cent.