The importance of genetic variation in the non-specific immune responses of Nile tilapia (Oreochromis niloticus L.) clones was investigated. Fully inbred clones (IC) of Nile tilapia, produced using gynogenesis and sex reversal, and crosses between these lines (outbred clones) were used in this study. Non-specific immune responses were compared between the ICs, including serum lysozyme activity and phagocytosis, and significant differences were observed between the different groups. Their natural resistance to Aeromonas hydrophila infection was also assessed by bacterial challenge. A positive correlation was observed between the level of infection obtained and the non-specific immune parameters measured. Cumulative mortalities of fish obtained in the study showed that when a IC susceptible to A. hydrophila was crossed with a resistant IC, the resulting progeny exhibited intermediate levels of resistance to that of their parents.