The cichlid fish T. leucosticta (Trewavas) was studied by fortnightly seining an area of an equatorial lake (Lake Naivasha) in Kenya to study their reproductive and gonadal patterns. The study lasted 16 months during which period temperature, rainfall and (for the last 11 months) sunlight patterns were obtained and analysed. Testicular and ovarian development was lowest in the months of July-August. No evidence of breeding was detected during this period. This period is also marked by the lowest rainfall, the lowest temperatures and least sunlight. Slight but fluctuating breeding activity and gonadal development is resumed in October following increased sunlight and temperatures from September. The period December to February is marked by sustained high temperatures, high sustained maximal sunlight and low rainfall. It is also the period of rising gonadal development in most mature males and females reaching a peak in both sexes in February. This is followed shortly after by a sharp peak in reproductive activity. It is concluded that sunlight and temperature are the most important factors influencing gonadal development and subsequent breeding in Tilapia even under equatorial conditions. When preceded by conditions of sustained high temperatures and good sunlight which are conducive to the maximal development of the gonads of both sexes, the onset of rainfall seems to stimulate the initiation of the peak breeding activity. Sustained heavy rainfall on the other hand seems to check breeding.