SEAFDEC Contribution No. 211.

Oreochromis niloticus fry reared in 50 L aquaria at a density of 1,000 fry manifested mortalities of 15% daily. Afflicted two-week old fry exhibited darker pigmentation, emaciation lesions, and surface swimming. Parasites and fungi were not observed upon direct microscopic examination of affected fry. Bacterial isolations from weak fry yielded the predominant growth of Pseudomonas sp. Subsequent pathogenicity experiments showed that Pseudomonas sp. is pathogenic at a dose of 10 super(7) cells/ml rearing water but not at a does of 10 super(6) cells/ml rearing water. By and large, the presence or absence of feed during infection did not affect virulence of the test bacteria. Antimicrobial sensitivity tests revealed sensitivity of Pseudomonas) sp. to Chlortetracycline, Colistin, Kanamycin, Oxytetracycline and Polymyxin B, and resistance to Nitrofurantoin and Sulfamethoxazole trimethoprim.