Entomological studies on the effect of water physical quality and water level change on thernoccurrence of Anopheles mosquito larvae and the formation of larval breeding habitats werernconducted in two villages (Ejersa and Kuma) at the Koka Reservoir in Central Ethiopiarnbetween August and December 2007. Data on the type and number and physical characteristicsrnof Anopheles larval breeding habitats, species composition and densities were recorded.rnMeteorological and reservoir water level data were compared with availability of Anophelesrnlarval breeding sites and densities. Four-year retrospective clinical data indicated that thernpattern of malaria transmission in the village at close proximity of the Koka reservoir isrnstrongly associated with reservoir water level change during the peak malaria transmissionrnseason. Data from adult and larval collection showed that Anopheles pharoensis, An. gambiaerns.l., An. coustani and An. squamosus were found in the study area in different proportions. An.rnpharoensis larvae were dominant at the village close by the reservoir while An. gambiae s.l.rnmore common at the second village away from reservoir in the short breeding interval duringrnthe study periods. The total count of An. pharoensis larvae at the reservoir site wasrnsignificantly higher than at the control village (X2 = 942.8, df = 1, p < 0.05). This indicates thatrnthis species prefers breeding sites created in association with shoreline puddles that providernideal turbid breeding pools with much floating aquatic vegetation. The total count of An.rngambiae s.l. at the reservoir site was also significantly higher (X2=200.5, df=1, p < 0.05) thanrnat a the nearby control village. Generally, mean larval density of An. gambiae s.l. was higher inrnslightly turbid and shallow aquatic habitats (F=16.97, p