(Izmid, Kocaeli), a city and port in northwestern Turkey, on the bank of the Gulf of İzmit in the Sea of Marmara; administrative center of Kocaeli Vilayet. Population, 123,000 (1970). A railroad and highway from Ankara to Istanbul run through the city, İzmit produces cellulose and paper, chemicals, tires, and piping. Food is processed in the city. It has an assembly plant and an oil refinery; there are large oil reserves in the area. Lead and zinc are concentrated nearby. Turkey’s major naval base is situated in Golcuk, a suburb of İzmit.