Petroleum product pipelines in India are found to contain a large number of various types of microorganisms either aerobic or anaerobic and these microorganisms either directly or indirectly enhance corrosion. Field studies have been carried out by CECRI to investigate the corrosion problem in a cross-country pipeline transporting petroleum product (diesel) for about 1400 km. fleterotrophic bacteria, iron bacteria, manganese oxidizers, acid producers were enumerated and identified in the pipeline whereas SRB could not be noticed in the pipeline. The reason for the absence of SRB is explained in the present study. Moreover, the interaction between heterotrophic and chemolithotrophic bacteria is explained with FTIR and NMR analysis. Influence of inhibitor on microbial corrosion has been investigated and the importance of selection of inhibitor has also been explained.