This work aimed at developing a procedure for spore quantification. Spore content was determined by analyzing dipicolinic acid (dpa) extracted from aerobic granules by 13 methods. Concentrated HC1 was able to release dpa completely. Results showed that dpa constituted 33.7 mg per g SS, meaning that about 337 mg per g SS were spores, not the normal vegetative cells. (C) 2007 Elsevier Ltd. All rights reserved.