An original method of breeding Phlebotomus papatasii (Scop.) in the laboratory in the Egyptian climate was evolved, whereby the development of a fungal growth, a well-known obstacle, was prevented by incorporating, in the larval feeding medium, living material from cultures of a bacterium, Alcaligenes viscosus, which had been found to be the predominant micro-organism in samples of soil collected from breeding places of P. papatasii in the field. Later, eight other Schizomycetes were also found to keep in check the development of mould. Certain fungicides also gave satisfactory results. Breeding pots made of unglazed earthenware were used. A layer of finely sieved sand (18 meshes) about 5 cm. deep was placed at the bottom of the pot and moistened with sterile water. To a mixture of one part dried, ground, sieved guineapig faeces (60 meshes) and two parts sieved dry earth (60 meshes), sufficient of a water suspension of the bacterium was added to impart a damp but friable condition. Granules of this larval feeding medium were then scattered over the sand in a loose layer about 1 cm. thick. Each breeding pot was stood in a petri dish of water and was placed in a small muslin cage to which adult insects were introduced. The females, which obtained their blood-meals from three-day-old white mice, oviposited within the pots, and no eggs were laid elsewhere. Development from egg to adult was completed successfully in this medium, and a healthy culture of the insect was maintained in the laboratory for several years.