A study was conducted to determine what methods could he us~'d' to' (l~se!>s, thernage and grol'/th of .Q. niloticus in Lake Awassa, Ethiopia. Furthermore, factorsrnresponsible for the, seasonal growth cycle of the fish vlere investigated.rnOtoliths (sagittae) and scales I'lere examined in relation to seasonal variationrnin some biological and environmental parameters.rnOto 1 i ths and scales ~ere found to form trans 1 ucent zones and checks twi ce arnyear, during January to February and June to July. Growth can, therefore, bernslow ,in this period. Spawning activity, decrease in water temperature and thernprobab 1e decrease in the Qu,an t ity of food consumed dur i n9 spawni n9 and thernpoor qual ity food consumed during these months v/ere considered to bernresponsible for slol'l growth. Otoliths and scales showed deposition of nellrntiSSU2 during f.iarch to nay and August through to D2cember. Growth viasrnbelieved to resume at a relatively faster rate in these months due torncomD1ption of spawning, and increase in water temperature. The quantity andrnquality of the food consumed by the fish could also increase during periods ofrnfast growth.rnThe first annulus in otoliths could not be identified. Therefore, averagern1engths-at-re1ative age were calculated for fish with 6, 8, 10, 12, 14 and 16rntranslucent zones in their oto1 iths. These data are each one year older andrnwere used to estimate the parameters of the von Gertalanffy growth equation.rnLoo was 30.0 cm for the males but, 29.4 for the females. K was 0.46 for thern,rnmales and it was 0.38 for the females. The theoretical age, to' 'las found tornbe 0.15 year for the males and -0.08 year for the females.rnRelative linear growth rates (S) estimates from otolith method, from lengthfrequencyrnmethod and from marked fi sh closely agre8d. Thi s verifi ed thatrnresults obtained from the otolith method could be used to describe the averagerngrowth pattern of O. niloticus population in the lake.rnThe sixth translucent zone in otoliths was believEd to be formed at the end ofrnthe first year of life. Ages estimated using Gulland's(1969) method providedrnsupportive evidence. Therefore, the 6th to the 16th trans 1 ucent zonesrncorrespond to the J.st to the 6th year of life, respectively. Ages werernassigned based on this finclin~. This indicated that more rapid growth inrnfavour of the males was apparent at and after the second year of life.rnLength-frequency distribution of experimental commercial catch indicated thatrnthe commercial gillnet cau9ht fish in their second year of life and older.rnThis gear does not catch immature fish.