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The inhibitive/adsorptive behavior of Costus afar leaves extract on the growth of sulphate reducing bacteria and anaerobic corrosion of pipeline steel caused by sulphate reducing bacteria were studied using gravimetric and media absorbance examination techniques. The test organism Desulphurvibro specie was isolated from corroded pipe line steel supplied by Altelerae Deconstriea Materae Nigeria ltd. Postgate medium was used for the isolation of the organism, and the coupons used for the study were prepared from the rusted pipeline steel. The absorbance of the media values decreased in the media containing Costus afar leaves extract from the absorbance values of the blank uninhibited medium. The decrease in the absorbance level has been suggested to be attributed to inhibition of Sulphate reducing bacteria in the inhibited media which resultantly led to light passing through with less interactions than the absorbance observed in the blank medium. The gravimetric results showed decrease in weight loss and corrosion rate in the medium containing Costus afar leaves extract from the blank, and the decrease in corrosion rate was more pronounced as the concentration of the Costus afar leaves extract increased in the media. Inhibition efficiency of the plant extract increased with increase in extract concentration. Besides obstructing the metabolic process of the microbial influenced corrosion agents (sulphate reducing bacteria), the plant extract molecules got absorbed to the pipeline steel surface serving as a protective shield against the invading microbial influenced corrosion agents (sulphate reducing bacteria). Adsorption of the extract molecules fitted into Langmuir adsorption isotherm.