Production of ultra low sulfur diesel through biodesulfurization process has been studied in a trickle bed reactor having a diameter of 0.066 m and a height of 0.6 m. Rhodococcus sp.(no. 2891 NCIM, Pune) in immobilized form has been used to desulfurize hydrotreated diesel having sulfur concentration in the range of 200-540 ppm. The initial substrate concentration, recycle ratio and volumetric flow rate have been chosen as process parameters. Excess biomass accumulation, which is reflected by the increment of pressure drop within the reactor, has been avoided by backwashing technique. Backwashing time has been correlated to volumetric flow rate. A deterministic mathematical model based on the growth kinetic parameters, namely, maximum specific growth rate mu(max) saturation constant K-s and yield coefficient Y-A/B, determined using systematic batch studies has been developed. Simulated data have been compared with the experimental results, which signify that the model predictions can explain the reality satisfactorily. (c) 2007 American Institute of Chemical Engineers.