Biomass of the blue–green alga Spirulina maxima was converted to methane using continuous stirred tank digesters with an energy conversion efficiency of 59%. Digesters were operated using once‐a‐day feeding with a retention time (θ) between 5 and 40 days, volatile solid concentrations (S_to) between 20 and 100 kg VS/m³, and temperatures between 15 and 52°C. The results indicated a maximum methane yield of 0.35 m³ (STP)/kg VS added at θ 30 days and S_to 20 kg VS/m³. Under such conditions, the energy conversion of the algal biomass to methane was 59%. The maximum methane production rate of 0.80 m³ (STP)/m³ day was obtained with θ= 20 days and S = 100 kg VS/m³. The mesophilic condition at 35°C produced the maximum methane yield and production rate. The process was stable and characterized by a high production of volatile acids (up to 23, 200 mg/L), alkalinity (up to 20, 000 mg/L), and ammonia (up to 7000 mg/L), and the high protein content of the biomass produced a well buffered environment which reduced inhibitory effects. At higher loading rates, the inhibition of methanogenic bacteria was observed, but there was no clear‐cut evidence that such a phenomenon was due to nonionized volatile acids or gaseous ammonia. The kinetic analysis using the model proposed by Chen and Hashimoto indicated that the minimum retention time was seven days. The optimum retention time increased gradually from 11 to 16 days with an increase in the initial volatile solid concentration. The kinetic constant K decreased with the improvement in the digester performance and increased in parallel with the ammonia concentration in the culture media.

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