We studied Double-toothed Kites (Harpagus bidentatus) in tropical lowland forest at Tikal National Park, Petén, Guatemala, documenting behavior and diet during the incubation and nestling periods. These 200-g kites are Accipiter-like in form and strikingly size-dimorphic for a kite. Modal clutch size was two, producing 0.63 fledglings per nesting attempt and 1.25 per successful nest. Nesting was largely synchronous among pairs, with hatching during the first month of the rainy season and fledging one month later. Incubation lasted 42–45 days and nestlings fledged at 29.5 days on average. A radio-tagged fledgling was fed near the nest for 35 days; 6–8 weeks after fledging it dispersed at least 10 km, presumably reaching independence. Males did not incubate or brood, and rarely fed nestlings directly. Males typically provided most but not all prey (mainly lizards) during incubation and early nestling periods. Insects in the nestling diet increased through the nestling period as females increasingly hunted, often bringing in insects. These kites hunted from perches, below and within the closed canopy of tall, mature forest, taking 60.5% insects, 38.1% lizards, and 1.4% other vertebrates; vertebrates comprised at least 75% of prey biomass. Most prey were taken from vegetation, but prey in flight also were captured. Active, adjacent nests averaged 1.35 km apart, for a maximum density estimate of 0.60 pairs km⁻² and a more likely estimate of 0.33–0.50 pairs km⁻² in homogeneous, favorable habitat and 0.29–0.44 pairs km⁻² for Tikal National Park as a whole.