The authors are to be commended for their desire to elucidate selector behavior using full-scale plant data. As indicated by the authors, full-scale application of selectors to control filamentous bulking phenomena in the municipal wastewater sector has not always achieved the expectations of earlier fundamental investigations at bench scale. They have concluded from anoxic selector investigations at five different full-scale municipal activated-sludge plants, that both anoxic and aerobic solids retention times (SRTs) are more appropriate variables to control than organic loading (food-to-microorganism ratio, F:M) to minimize the growth of filamentous organisms.