This study was conducted in order to evaluate the potential benefit of Bacillus pumilus and a commercial product (‘Organic Green’^TM) as a probiotic in the culture of the tilapia nilotica. Two doses of B. pumilus (10⁶ and 10¹² g^-1 diet fed) and Organic Green^TM (1 and 2 g kg^-1 diet fed) were used as feed additives and administered for periods of 1 (groups 2, 3, 4 and 5 respectively) and 2 (groups 6, 7, 8 and 9 respectively) months; group 1 served as a control. Each group consisted of 4 equal replicates of 320 fish. Body weight, nitroblue tetrazolium (NBT) activity, haematocrit values and total and differential leucocytic counts (TLC and DLC) were recorded in each group after 1 and 2 months. At the end of the experiment (8 months), survival and individual body weights were recorded. Challenge infections were performed after 1, 2 and 8 months using 0.5mL culture suspension of a pathogenic reference strain of Aeromonas hydrophila (10⁸ bacteriamL^-1). The NBT values were increased significantly in all treated groups, except those administered B. pumillus (groups 3 and 6). There were significant changes in haematocrit values and TLC and DLCs in the treated groups, except group 3, which showed a significant increase in TLC, lymphocytes and monocytes. There was a statistically significant increase in the individual body weight in groups 5, 6, 8 and 9. The survival rates of all treatment groups were higher than the control group, the differences being statistically significant in groups 3 and 9. The challenge infection showed a variable response with the type and dose of treatment and the period of application. The highest relative levels of protection were an average increase of 62.5% after 1 month (groups 3 and 5), 55.6% after 2 months (group 7) and 15.8% after 8 months (groups 8 and 9). Overall, both types of probiotics induced a similar effect. The results show the potential of using probiotics to enhance immune and health status and improve disease resistance in Oreochromis niloticus, thereby improving growth performance. However, further extensive testing, including a full commercial cost-benefit analysis, is necessary before recommending their application in aquaculture.