This report describes results of field studies on Phlebotomus sandflies from August 1962 to October 1963 in the Paloich area, Upper Nile Province, Sudan about 600 km south of Khartoum. A description of the area is given which includes topography, climate, flora and people. The ecology of the 13 Phlebotomus species of the area is summarized and habitat preferences, seasonal occurrences and hosts are described. The changes of the sandfly fauna from the wet to the dry season are discussed and it is concluded that the change is one of addition and subtraction, but not species replacement. Three species of the area, orientalis, papatasi and heischi, frequently bite man and two species, clydei and schwetzi, rarely do so. From the medical viewpoint, P. langeroni orientalis is the most important form as it is the probable vector of kala-azar. The flight range of orientalis, determined by marking flies with fluorescent powders, was at least 730 m. Some flies were also found to reach 1.5–2 m above the ground in free flight. The effects of wind on the man-biting activity of orientalis were studied and it was determined that winds below 1.5 m/sec (5.3 kmph) are not deterrent, but between 1.5 and 2.5 m/sec biting activity diminishes and at 4.0 m/sec (14.5 kmph) ceases almost entirely. Average time required for feeding to repletion on man is 4 min 35 sec. A host list has been compiled from cage tests and from the literature. Cavities in trees are the chief daytime resting spots in the wet season; in the dry season soil cracks are used in the day to escape the heat and dessication of above surface locations and foliage of two forest evergreens harbor flies at night. Two and one-half tons of soil were searched for immature stages by direct examination and a sugar flotation technique, but only a single larva of P. africanus was found in mud at the base of a tree. The species are placed in two subgenera, Phlebotomus and Sergentomyia. Keys, descriptions and illustrations are given for each species and new synonymies are proposed for some named varieties.