The design of the study was to evaluate the antimicrobial activity of Allium cepa in cultured Clarias gariepinus. The proximate compositions and Mineral assay of whole A. cepa bulb and experimental diets were determined using standard methods. Microbial susceptibility assay was carried out in vitro on Staphloccocus aureus and Escherichia coli using Agar well diffusion. Bacterial isolation and identification from the different treatments and control was carried out according to standard methods. High moisture content (89.25 %) was recorded followed by carbohydrate (9.45%) and crude protein (7.21%). Experimental and control diets revealed 50 % and above crude protein content. In vitro susceptibility test of the various onion extracts and antibiotic shows susceptibility of the reference strains to the onion extracts. Bacterial isolation and identification showed the presence of pathogenic gram positive and negative coli and bacilli respectively. Results obtained for total fungi counts show the presence of both systemic and superficial fungi. Lower bacteria and fungi counts were observed 14 days after the withdrawal of experimental diets. Conclusively, onion can be used as antimicrobial agent in the culture of Clarias gariepinus
