SUMMARY.Lake Dakataua (5°S, 150°E) is a large freshwater lake which fills the caldera at the tip of the Willaumez Peninsula on West New Britain, Papua New Guinea. A peninsula produced by post‐collapse volcanic activity divides the lake into two basins connected by a narrow channel. The surface of the lake isc. 76 m above sea level, the surface area is 48 km2, and the maximum depthc. 120 m (Lowder&Carmichael, 1970). A bathymetric map of the lake has been constructed from fathometer transects.In October‐November, 1974, the lake was alkaline throughout with surface pH 7.6–8.2; acidity increased with depth to pH 7.1–7.5. Surface temperatures were 30.8–31.9°C. There were thermoclines at 22 m and at 40–45 m. Minimum temperature recorded was 26.8°C (at 80 m). Oxygen saturation curves were similar to the temperature curves with sharp gradients at 22 m and 40–45 m. There was no measurable O2from 80 m downward. Living organisms were common in dredge hauls to 20 m but were not found in those from greater depths. Carbon dioxide concentration rose steadily from 1.4 mg I−1at the surface to 19.6 mg I−1at 80 m. Average Secchi disc transparency was 11.1 m.Shallow water areas support dense beds of mixed aquatic plants (Najas tenuifolia and Charasp.). Invertebrates collected included two species of sponge, a rotifer, an ostracod, six species of molluscs, seven species of Cladocera, a copepod, eight species of Hemiptera, two species of Trichoptera, ten species of Odonata, two species of Coleoptera, and seven species of Chironomidae. Vertebrates included frogs (two species) and crocodiles. Water birds, including ducks, grebes, and waders, were abundant. The biota of L. Dakataua is more diverse than that of nearby Lake Wisdom probably due to the combination of greater age and greater proximity to sources of colonists. Most species found in L. Wisdom are also