Green algae are able to convert solar energy into biological hydrogen (solar-to-H-2) from H+ via chloroplast hydrogenase (HydA), which can accept electrons directly from ferredoxin and generates H-2. HydA is nuclear-encoded by the hydA gene, and its transcription and activity can only be observed under anaerobic or sulfur-deprived conditions. In this study we attempted to homologously overexpress hydA to enhance H-2 production in Chlorella sp. DT (DT) under aerobic and S-supplied (+S) conditions. The nucleotides of partial genomic DNA and cDNA of DT hydA were sequenced and cloned. The engineered coding region of the hydA gene (hydAc) was constructed into plasmids and driven by the promoters that can function under aerobic conditions. The DT cells were transformed with the constructed plasmids and the resultant transgenics were verified as containing homologous hydAc fragments in their genomic DNAs. Under aerobic and +S conditions, the RNA transcription of hyAc and the protein expression of HyA in the alga DT transgenics could be observed. The H-2 production of the DT transgenics with homologously overexpressed HydA increased by as much as 7 to 10-fold in comparison to the DT wild type under the same conditions. Copyright (c) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.