The adaptation of activated sludge from the Hajdow sewage treatment plant in a laboratory SBR was studied. The structure of the ciliate assembly was considered as a criterion. 32 ciliate species were found during the experiment. The composition and changes in the ciliate community structure during the process of activated sludge adaptation was examined. In the process of adaptation, reduction was observed in the number of ciliate species together with an increase in assembly total abundance. The decrease in the Shannon diversity index and equitability index in the adaptation process was observed. In the process of adaptation, two states of ciliate assembly were marked out - unstable transient period and stable period. During the transient period, reduction of ammonium utilization efficiency down to 50% and its subsequent increase up to 80% in the stable period were observed. In the transient period, the Simpson dominance index remained low but increased in the stable period. At a temperature of 10°C, the transient period lasted from six to nine days. After the stabilization process, the diversity of the ciliate assemblage remained at a lower level. Rarefaction methods showed that the number of potential ecological niches of ciliate amounted to 30 in the adaptation period, whereas there were only 15-20 ecological niches in adapted sludge.