A simulated fuel storage tank was used to study biodeterioration of linear low-density polyethylene over 100 days. The system consisted of a water layer inoculated with microorganisms and a fuel layer of diesel/biodiesel. Biodeterioration was characterized measuring: biofilm growth, surface chemistry, crystallinity, and topography. Results showed greater accumulation of biofilm at higher biodiesel concentrations. Polyethylene biodegradation measured as consumption of oxidized species, increase in contact angle with water and reduction in electron donor groups was observed in all samples and was slightly higher in biodiesel-rich fuels. Topography changes and weight loss showed that microbial penetration in the polymer was superficial.