This study assessed the Length-Weight relationship and reproductive biology of Oreochromis niloticus (Linnaeus, 1757) in Lake Shala, Ethiopia. A sample of 343 fish from 7.70 cm to 33 cm Total Length (TL) and from 7.80 g to 708.21 g Total Weight (TW) were collected between January and December, 2018 using a gill net. The Length-Weight relationship was curvilinear (TW = 0.0104TL3.19, R2 = 0.98. n = 343) and indicating positive allometric growth of O. niloticus. The sex ratio (0.93:1) was not significant from the hypothetical distribution of 1:1 (?2 =0.47, P > 0.05). The Condition Factor was 1.04, 1.06 and 1.05 for male, female and combined sexes, respectively and was not statistically significant (P > 0.05). Rainy (July) and dry (February) seasons were determined as the breeding period and the fecundity was significantly correlated with TL (F = 0.56TL2.29, R2 = 0.93, P < 0.05) and TW (F = 18.83TW0.67, R2= 0.90, P < 0.05). The fecundity ranged between 240 and 1,642 eggs per fish, with a mean fecundity of 806 eggs. In conclusion, the study found that O. niloticus exhibited positive allometric growth pattern and breeds throughout the year, with their peak breeding time correlated with rainy seasons.

Keywords: Allometric growth, Breeding, Condition Factor, Fecundity, Sex ratio