Infectious fish diseases are among the known contributing factor in reduced productivity of fish farming enterprises. Despite of the growing importance of global fish farming industry, research in fish and other aquatic stocks relevant to Tanzania is limited. This paper presents preliminary results of the ongoing investigation on fish mortalities which occurred in fish farm located in Kibaha District. The paper also present preliminary results of formalin fixed samples received from other parts of Tanzania. In all the samples; branchitis, gill deformity, and intracellular chlamydia like organisms were the major findings regardless of the source. Other findings include encysted trematode metacercaria in different anatomical locations accompanied with variable pathomorphological changes to the host tissues. Interestingly, mortalities ceased in the affected farm after replenishment of water supply suggesting that either poor water quality was the main predisposing factor or aggravated the observed disease conditions. Therefore maintenance of water quality and or water replacement is recommended as the first intervention measure where poor water quality is strongly suspected to be associated with mortalities in fish farms. Further studies on the pathobiological characteristic of the observed infectious organisms will provide more insights on the suspected relationships between the environmental factors in one hand and progression of the observed pathological changes in fish.