Radiotelemetric monitoring of 18 free-ranging frillneck lizards in Kakadu National Park, combined with dissection of museum specimens, provided extensive data on the natural history and general ecology of this large and spectacular lizard. Activity patterns and reproductive cycles are highly seasonal, with lizards inactive during the drier (and cooler) months of the year. Frillnecks are primarily arboreal and are relatively unselective with regard to the species of tree used except that one common species, Eucalyptus confertiflora, is avoided. Telemetered lizards usually clung to branches high in the canopy. During the mating season (November-December), males had larger activity ranges than did females (means of 2-5 vs 0-7 ha) and made longer daily movements (means of 69 vs 23 m). Frillneck lizards bask briefly in the morning, but body temperatures follow ambient temperatures for most of the day. Thermal heterogeneity in the lizards’ habitat is low, so opportunities for behavioural regulation (especially reduction) of body temperature are limited. Body temperatures are highly correlated with air temperatures and are often close to 40°C. Female frillnecks are considerably smaller than males, and they produce 4-13 eggs during the wet season. Chlamydosaurus are apparently unique among reptiles in using bipedal locomotion during routine foraging. These lizards are ‘sit and wait’ predators, descending from arboreal vantage points to seize lepidopteran larvae or massed swarms of hymenopteran or isopteran alates. Many different types of insects are eaten, but vertebrate prey items are rarely taken.