A pilot-scale PHA microbial enrichment reactor fed on OFMSW leachate was monitored in order to assess the influence of free ammonia nitrogen (FAN) on its performance. The enrichment reactor consisted of a SBR with a 12-hours cycle which included a feast phase, a settling phase and a famine phase with external nitrogen addition. Based on the microscope pictures and FISH analysis, at least two different PHA producing bacteria were identified: small (< 2.5 μm) and big PHA producing bacteria (< 5 μm). Big PHA bacteria appeared from FAN concentrations higher than 50 mg/L, but concentrations higher than 150 mg/L were highly toxic for the whole PHA microbial enrichment. Finally, it was found that a FAN concentration around 60-70 mg/L might produce a microbial enrichment with good settling properties (SVI30 < 80 mL/g and BLAS < 5%) and maximum PHA yields (around 0.50 g PHA/g sCOD), possibly due to the presence of big PHA producers which are assumed to be more efficient and heavier after feast. However, this FAN concentration may lead to a lower settleable biomass production yield (around 0.30 g VSS/g sCOD). Since these results are not conclusive, it is suggested to test the observations of this pilot study in lab-scale experiments to evaluate the potential of FAN as an additional selective pressure in a PHA microbial enrichment.