The antidiarrhoeal activity of 11-Octadecenoic acid, methyl ester isolated from the seeds of Acacia nilotica was determined using standard methods. The Phytochemical screening and the antidiarrhoeal screening of the crude extracts were determined and compared to the isolated fraction. The fraction was isolated by directing the fractionation of ethyl acetate extract of the air dried seeds with microbial sensitivity test. The result of the phytochemical screening showed that the seeds of Acacia nilotica Linn contains alkaloids, saponins, cardiac glycosides, anthraquinones, steroids, flavonoids, phenols, tannins, terpenoids and triterpenoids. The results of the antidiarrhoeal screening showed that the ethyl acetate extract of the seeds of Acacia nilotica Linn exhibited the highest activities against the test microbes with zones of inhibition diameter ranging from 27-32 mm against Salmonela typhi, Escherichia coli, Streptococcus feacalis, Staphylococcus aureus, Candida krusei and Shigella dysentriae and was subjected to activity guided isolation, leading to the isolation of fraction 21. The structure of the fraction was identified from 13CNMR, 1HNMR, IR and GC-MS spectral data. The isolation, structural elucidation, NMR spectral assignment and bioactivities were reported.