Effect or nitrogen on methanogenesis of phenol has been evaluated. Methane generation rate rate constant (k) has been found to vary from 0.12 to 0.21 d(-1). The process was not impaired due to reduction of COD:N from 300:10 to 300:1. However, a definite decreasing trend in k was noticed at COD:N<300:1. The mu(b) at 90 days (mu(b90)) was reduced from 0.94 to 0.12. Gas generation completely ceased at COD: N equal to 300:0. The trend of sludge activity was also found to be similar to that of k and mu(b90). The variation of COD:N from 300:1 to 300:0 in a UASB reactor, did influence the conversion of phenol COD to methane COD i.e. the (i) CH4-COD decreased from 90 to 40%, and (ii) cell yield reduced to 25-50%. The average cell yield was 3.5%. Percent N in cells varied from 10 to 14%. The activity of the sludge assessed as SMA was found in the range from 0.15 to 0.66 g CH4-COD/g VSS/d. The optimum COD:N for phenolic wastewater from both bench-scale batch and continuous reactor study has been estimated to be 300:1. (C) 2009 Elsevier B.V. All rights reserved.