Ammonium removal from artificial wastewater by microalgal-bacterial consortia in a flat-panel reactor (FPR1) was compared with a microalgae-only flat-panel reactor (FPR2). The microalgal-bacterial consortia removed ammonium at higher rates (100 +/- 18 mg NH4+-N L-1 d(-1)) than the microalgae (44 +/- 16 mg NH4+-N L-1 d(-1)), after the system achieved a stable performance at a 2 days hydraulic retention time. Nitrifiers present in the microalgae-bacteria consortia increased the ammonium removal: the ammonium removal rate by nitrifiers and by algae in FPR1 was, respectively, 50 (+/- 18) and 49 (+/- 22) mg NH4+-N L-1 d(-1). Apparently ammonium removal by algae was not significantly different between FPR1 and FPR2. The activity of the nitrifiers did not negatively affect the nitrogen uptake by algae, but improved the total ammonium removal rate of FPR1.