The study was undertaken to investigate the effects of rainy and dry periods on the gross and histology of the testes of Oreochromis niloticus in Zaria, Kaduna State, Nigeria. Twenty-four adult wild tilapia fish with average weight of 234.74±5.20 gm and 177.84±31.07 gm was used during rainy and dry periods, respectively. They measured a standard body length of 23.50±0.29 cm and 22.17±0.73 cm during rainy and dry periods, respectively. The fishes were sourced from Zaria dam and transported live in open plastic troughs containing clean water to the Gross Anatomy Laboratory in the Department of Veterinary Anatomy, Ahmadu Bello University Zaria. Each fish was euthanized using tricaine anaesthetic at 8 drops/litre of water. The testes were extracted, weighed and fixed in bouin’s fluid for 24hours. Each testis was then dehydrated through series of ascending concentrations of ethanol, cleared in xylene, embedded in paraffin wax and sectioned at 5µm. Hematoxylin (H) & Eosin (E) were used for histological evaluation. Grossly, the testes of O. niloticus were whitish, non-lobulated and well developed during both rainy and dry periods. Each testis had a smooth lateral and medial edge. Seminal vesicles and Bulbourethral glands were absent. The average weight of both testes shows insignificant variation across both periods (0.458). The Gonado-somatic index value was insignificantly higher during rainy period (0.159) compare to dry period. Histologically, the lumen of seminiferous lobules was filled with spermatozoa during both rainy and dry periods. The tunica albuginea and interstitial connective tissues were thin across both periods. In conclusion, Gross and histological results which coincide with the values of Gonado-somatic index revealed that wild adult tilapia fish spawns during both rainy and dry periods in Zaria, Nigeria. Keywords: Gonado-somatic index, Testes, Seminiferous lobules, Spermatozoa, Wild Tilapia, Zaria dam