Summary1. Lake Rudolf, nothern Kenya, has one of the largest undisturbed populations of the Nile Crocodile. In 1965 the Kenya Game Department initiated the Lake Rudolf Crocodile Research Project. Central Island, where the crocodiles live under undisturbed conditions, was chosen for studying breeding behaviour and ecology of the reptile. Eight months were spent on the island.2. On the island most of the crocodile population was confined to a crater lake, Lake C, which supported up to 500 animate at the height of the 1965‐66 breeding season.3. The 1200 m shoreline of Lake C was shared out between about a dozen large males each guarding his territory by patrolling up and down the shore. The territorial shore lengths ranged from 60 to 230 m. The territories extended about 50 m into the water.4. The crocodiles were first seen courting on 10.10.65 in Lake C. The territorial males exhibited a “courtship splash display”. Copulation ranged from 30 to 100 seconds; eleven copulations averaged 58 seconds.5. The factors important in site selection for nesting are shade, suitable soil, proximity to water and the degree of slope of the shore. Because the shores of a second crater lake, Lake A, satisfied these conditions they had the largest number of nests. The fully exposed sites on Lake C, and the Lake Rudolf shores, had very few nests.6. The females dig the nests with their forelimbs, using them in turn. The hind limbs and the belly are used to push away the soil collecting at the mouth of the burrow. The female guards the nest constantly throughout the incubation period of three months against monitor luzards. Laying began in the third week of November, 1965 reaching its peak during the second week of December. The crocodileeggis oblong, measuring 55.5 to 89.0 mm in length and 43.0 to 54.0 mm in width. Mean weights of the eggs from 15 clutches ranged from 83.7 to 126.6 g. Clutch sizes on Central Island varied from 14 to 46 eggs, giving a mean of 33 eggs per clutch.When the young are about to hatch, the mother releases them by digging up the nest. The young when they hatch are about 31.0 cm long and weigh about 76.8 g. They are guarded by the mother for at least six weeks. Infant mortality is probably very conside