Entomological studies were conducted in Tanga region, Tanzania. over a two year period, to assess the effect of various environmental features and larval population density on the productivity of An. gambiae s.l. breeding sites for adult mosquitoes. These studies included: (i) Nineteen rounds of systematic dipping for larvae in about 150 mapped potential breeding sites around two villages. During this time records were kept for each potential breeding site of the dominant plants, suspended mud, odour, shade, depth, speed of water-flow, nature of margin, surface particles, and other features. The insects suspected of feeding on mosquito larvae were recorded. These included members of the family Notonectidae, Culex tigripes, and dragon- and damsel-fly larvae. (ii) Field experiments to assess the effect of different larval densities on the emergence of adult Anopheles gambiae s.l. (iii) Quantitative estimation of productivity of adult An. gambiae s.l. per unit area of water. (iv) Field monitoring of the relationship of water depth and the production of first instars of An. gambiae s.l. (v) Investigation of the impact of larviciding with temephos and Bacillus thuringiensis israelensis around one village on the larvae and on the density and parous rate of the adult female mosquito population in comparison with untreated controls. For these studies adult mosquitoes were trapped weekly using CDC light traps hung beside untreated bednets. (vi) Mark-release-recapture to quantify immigration of An. gambiae s.l. from other villages into the one in which larviciding was carried out.