Sunlight represents an important choice for low cost photosynthetic cultures irradiation, especially in up-scaled processes. The aim of this study was the assessment of the hydrogen production performances of Rhodopseudomonas palustris strain 42OL cultivated outdoors, under sunlight, in a 50 L tubular photobioreactor. Moreover, bacteriochlorophyll a fluorescence was used for the first time as a tool for monitoring the physiological status of Rp. palustris mass cultures. The maximum H-2 production rate of 27.2 ml L-1 h(-1), and the mean rate per illuminated surface of 3.54 +/- 1.53 L m(-2) d(-1) obtained resulted the highest so far reported for outdoor experiments carried out with purple bacteria in tubular photobioreactors of comparable dimensions. The in-situ measurement of bacteriochlorophyll a fluorescence (F-v/F-m) showed that the photosystem was unaffected by changing light irradiance during the day and remained fully active over the whole period of hydrogen production. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.