Isochrysis galbana and Selenastrum capricornutum, marine and freshwater microalgae species respectively, were co-digested with sewage sludge under mesophilic and thermophilic conditions. The substrates and the temperatures significantly influenced biogas production. Under mesophilic conditions, the sewage sludge digestion produced 451 +/- 12 mL(Biogas/gsv). Furthermore, all digesters were fed with 1. galbana, or mixed with sludge, resulting in an average of 440 +/- 25 mL(Biogas/gsv). On the contrary, S. capricornutum produced 271 +/- 6 mL/(Biogas/gsv) and in the mixtures containing sludge produced intermediate values between sludge and microalgae production. Under thermophilic conditions, the sewage sludge digestion achieved yet the highest biogas yield, 566 +/- 5 mL(Biogas/gsv). During co-digestion, biogas production decreased when the microalgae content increased, and for I. galbana and for S. capricornutum it reached minimum values, 261 +/- 11 and 185 +/- 7 mL(Biogas/gsv), respectively. However, no evidence of inhibition was found and the low yields were attributed to microalgae species characteristics. The methane content in biogas showed similar values, independently from the digested substrate, although this increased by approximately 5% under thermophilic condition. (C) 2014 Elsevier Ltd. All rights reserved.