This study examines the friction and heat transfer characteristics of water flow in internally roughened rectangular ducts. The low aspect ratio ducts have two- or three-dimensional roughness formed into the two wide dimensions of the tube wall. The ducts considered are representative of turbulator tube geometries used in low flow automotive heat radiators.Isothermal friction data were taken for Reynolds numbers, based on hydraulic diameter, in the range 200 to 11,000. The friction factor was measured for six rough tubes and a smooth tube. Heat transfer data were taken for one smooth and three rough tubes. Data were taken for Reynolds numbers between 3,000 and 10,000. A modified Wilson plot technique was used to reduce the heat transfer data. The heat transfer and friction characteristics of the turbulator tubes are compared to those of standard, plain tubes.