Visceral (VL) and cutaneous leishmaniasis (CL) are public health problems in the southwest of Saudi Arabia. The causative parasites, the vectors and the possible animal reservoirs In that area were all unknown before this study began. Because of the size of the area and to achieve the different aims of the study, a laboratory was established in Abha City which is in the centre of the study area, altitude approximately 2000 m. More than 50 isolates from CL lesions, 17 from VL patients and 700 human filter paper blood samples were collected. Eighty nine feral dogs were captured In Kala-azar endemic areas and examined for Leishmania. More than 8,000 sandflies collected from fixed stations were examined and more than 1600 Phlebotomus females were dissected. Forty four human isolates from human CL lesions were typed by the isoenzyme technique which showed that L. tropica is responsible for CL in both lowlands (altitude about 450 - 700 m) and the highlands (altitude about 2000 m). Only one zymodeme (LON-63) was found in the isolates from the lowlands but, in the highlands, four zymodemes (LON-10, 71, 72 and 73) were found. Zymodemes LON-10 and LON-71 were also Isolated from Phlebotomus sergenti, which has been shown clearly to be a major vector of L. tropica In the highlands. Animal susceptibility experiments showed. that neither BALB/c mice nor the golden hamster were susceptible to L. tropica. L. donovani sensu lato zymodeme LON-42 causes zoonotic Infantile Kala-azar in areas at altitudes of up to 700m. Neither the vector nor the reservoir host were identified in spite of an active search for them. The limited sero-epidemiological survey using the ELISA procedure revealed a high frequency of antibodies in children in Al Baha province, much greater than was previously believed to exist. Feral dogs In this area were found to be carriers of typical L. infantum, NOT the parasite found in man. The prevalence rate in dogs was high (19.3%). The dog's possible role in the epidemiology of Kala-azar in the study area is discussed. The entomological studies revealed that six species of Phlebotomus exist in the study area, with Ph. sergenti as the dominant species In the highlands and Ph. bergeroti in the lowlands. Some species such as Ph. arabicus are limited to high altitudes (about 2000 m), and others such as Ph. alexandri to low altitudes (up to 700 m). Ph. orientalis was found mainly in the highlands but a few samples were collected from the lowlands. Sergentomyi species were abundant in all areas. The seasonal distribution based on a longitudinal study indicated that the population peak in both ecological areas (high and lowlands) occurs in July. The factors Including the collection method and trap sites controlling the apparent seasonal distributions are discussed. Statistical data on the total cases of CL and VL reported In The Kingdom are presented and data from the study area are compared with those from other areas such as the Eastern Province (where L. major is dominant) to give an overall picture of the leishmaniases throughout the country.