The current study text is divided into two parts. Part I deals with taxonomy and ecological studies of the Culicidae (Diptera) and part II contains toxological observation on mosquito species with special emphasis given to Culex pipiens found in city of Abha, Asir region, Saudi Arabia. Part I: Twelve species spread over four genera of Culicidae mosquitoes were recorded from Abha city. They have been considered in detail with a particular reference to the distribution, diagnosis, and their vector competence. Some general aspects of anopheline behaviour are discussed. The species dealt in the present work are Anopheles arabiensis Patton, An. dthali Patton, An. multicolor Theobad, An. rupicolus Lewis, An. sergentii Theobad, An. Tenebrosus, Donitz, An. turkhudi Liston, Aedes sp., Culex pipiens L., Cx.quinquefasciatus Say, Cx. theileri Theobad, and Culiseta subochrea Edward. On ecological observations the main concerns is given to occurrence and abundance of mosquitoes. Seasonal fluctuation in relation to temperature, humidity and rainfall in different localities of Abha city (Abha Dam, Mahala and Al-Sharf) were..