The relationship between hydrogen generation and the age of culture was investigated under fed-batch growth conditions. The specific growth rate (mu(e)) was determined during the log phase of the growth curve and the mu(eMax) was 0.02643 h(-1). Boltzmann's sigmoidal regression model was used to determine the specific rate of hydrogen evolution (mu H): the maximum was 0.04440 h(-1). At low irradiance (36-75 W m(-2)), an inverse relationship was found between mu H and I; after increasing the irradiance further, mu H reached a plateau (0.00916 h(-1)). The maximum reactor yield of cumulative hydrogen (4.5 l) was obtained at an irradiance of 320 W m(-2), but the highest hydrogen evolution rate (17.217 ml h(-1)) was achieved at 500 W m(-2). The light conversion efficiency reached its maximum (6.91%) at the lowest irradiance investigated (36 W m(-2)); when the irradiance increased further, it decreased progressively down to 0.36%. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.