The nitrifying activity in mixed cultures enriched to accumulate polyhydroxyalkanoates (PHA) is in general inhibited by the addition of chemicals, such as allylthiourea (ATU). In the present study, a sequencing batch reactor was inoculated with a mixed microbial culture enriched in PHA-accumulating microorganisms, operating under complete aerobic conditions, feast/famine regime and with ATU addition. Genera Comamonas, Azoarcus and Thauera, all of them known as PHA-accumulating bacteria, were the dominant microbial populations present in the mixed culture. They showed a PHA storage capacity up to 60.0 wt%, as hydroxybutyrate (HB) and hydroxyvalerate (HV), in a HB: HV ratio of 63: 37. After 20 days of operation, ATU addition was stopped and the reactor was run under the same operational conditions for 400 days. The development of partial nitritation activity (ammonia oxidation to nitrite) was observed, presumably due to free ammonia inhibition or the selected sludge retention time. Ammonium oxidizing bacteria (AOB) from genera Nitrosomonas, Nitrosococcus and Nitrosospira were detected, whereas the composition of the dominant PHA-accumulating microbial populations was the same as in the inoculum. This enriched mixed culture reached a maximum PHA accumulation of 59.2 wt %, with a HB: HV ratio of 59: 41. Results showed the simultaneous occurrence of partial nitritation and the enrichment in PHA-accumulating microorganisms, which did not affect the maximum PHA storage capacity of the system.