A photobioreactor, its construction and a method for its use are disclosed. The photobioreactor comprises a shallow sloping (1:20) gas-tight first tank having a dam or weir 15 connecting it with a second gas-tight tank having an opaque sloping base and transparent roof and a second dam at the bottom of the slope. In use, media overflows from the dam of the first tank and down the sloping surface of the second tank as a thin film. The dams may be arranged adjacent plate 14 to form a narrow channel or opening 16. Preferably the second tank comprises barriers 24 (figure 3) to agitate the flowing thin film. The device further comprises filter/drier 29 (figure 4) which contains a mesh filter to separate biomass from culture medium (which is recycled) and a hot air drier to dry the biomass for subsequent biofuel extraction.
