A biological survey of 26 lakes of the western dune series of the North Island was made during the summers of 1949 and 1950. At each lake a shore-line and bathymetric survey was made and contour maps have been prepared. Temperature and transparency measurements were also taken. Analysis for essential chemicals, particularly oxygen, was made. Thermal stratification and deoxygenation of the hypolimnion was found in five lakes. Details of emergent and submerged aquatic plants have been included on the contour maps. Qualitative and quantitative samples were taken of plankton and bottom fauna. The present stocks of fish were examined and waterfowl were noted. All the lakes with an outlet supported eels. The Northland series contained no stocks of game fish, but in the Wellington series limited stocks of perch and trout occurred.