Tilapia, Oreochromis niloticus L. is the most important fish in Ethiopia.rnKnowledge on age and growth of the fish is essential for fisheries management,rnand for further detailed studies. In this study, age and growth of O. niloticus inrnLakes Zwa, Langeno and Chamo was studied from microzones and macrozonesrnin otoliths. The usefulness of otoliths to determine age was established byrnexamining the existence of regularly formed translucent and opaquernmacrozones. The effect of subtle changes in temperature on body growth andrnon otolith macrozone formation was also experimentally tested. The growth ofrnthe fish in various lakes was also compared.rnTwo translucent macrozones, each associated with biannulus (a type ofrnannulus formed twice a year), were fOlmed each year in otoliths of O. niloticusrnin Lake Zwai. Biannulus formation was connected to fluctuations inrntemperature, quality and quantity of food, and to intensive spawning activity.rnTranslucent macrozone was fOlmed in otoliths, and somatic tissue growthrnwas slow, in tilapia that were transferred from 240C to slightly lowerrntemperature treatments. Fish that were transferred from 240 C to higherrntemperatures grew rapidly, and more opaque material was deposited in theirrnotoliths. At the high test temperatures, translucent macrozone was formed inrnotoliths of fish that had dpening or dpe gonads.rnO. niloticus in Lake Zwai has two spawning peaks, hence two majorrnannual recruitment cohorts. Fish from the two cohorts were discriminated andrnage was determined from the number of biannulus in otoliths by relating medianrnhatch-dates with the dates of capture. Ages determined in this mannerrn(macrozonal ages) were highly correlated (r2 = 0.85) with microzonal ages.rnGrowth of juvenile O. niloticus in Lakes Chamo, Zwai and Langeno wasrndescribed by Gompertz model fitted to length and microzonal age data. Growthrnwas most rapid in Lake Chamo, intennediate in Lake Zwai and slowest in LakernLangeno. Rapid growth of juvenile O. niloticus in Lake Chamo was attributedrnmainly to warm temperature and better food quality in this lake.rnThe growth of adult O. niloticus in Lakes Zwai, Awassa and Haiq wasrndescribed by von Bertalanffy model fitted to mean length and macrozonal agerndata. Growth rate was most rapid in Lake Haiq, intermediate in Awassa andrnleast in Zwai. Rapid growth in Haiq was attdbuted to better quality food in this lake.