The effect of aquatic weed killer Reglone on 0. miloticus mitotic chromosomes was studied. Animals were exposed to water containing Reglone for 72 and 96 hours durations. Metaphases from intestine tissue were prepared, stained and scored for chromosomal aberrations. The results obtained showed that exposure of fish to water containing Reglone resulted in an enhancment in the frequency of metaphases exhibiting chromosomal aberrations (20.95% & 20.64% for 72 h. and 96 h., respectively virsus 6.5% for control fish). Single chromatid breaks and gaps were the highest aberrations compared to the gross chromosomal aberrations such as dicentrics and translocations. Other chromosomal abnormalities such as condensation, fragmentation and stickiness were observed in the treated aminals. The present results indicate the possbility of using chromosomal aberration analysis in 0.niloticus as a useful tool for studying potentially dangerous waterborne chemicals in vivo.