Isabela (ĭzəbĕlˈə, Span. ēsäbāˈlä) [key], province (1990 pop. 1,080,341), NE Luzon, the Philippines. The capital is Ilagan. The fertile Cagayan River valley, which is in the central and eastern part of the province, is a leading tobacco- and cacao-producing region. The rugged Sierra Madre on the east is mineral rich; iron, nickel, and copper deposits, as well as the nation's major manganese mines, are there.