This study investigated the fate of phthalates in a trickle filter sewage treatment works. A wide variety of phthalates were researched of which only two were present in significant amounts. Mean concentrations of di-(2-ethylhexyl)phthalate (DEHP) and diethyl phthalate (DEP) measured throughout the system were 23.6 and 25.0 mg/l in raw sewage, 22.0 and 24.8 mg/l in primary, 14.6 and 0.60 mg/l in trickle filter, 18.6 and 0.10 mg/l in humus tank and 18.5 and 0.40 mg/l in reedbed effluents, respectively. Removal by the trickle filter was constantly high for DEP (94–99%) whereas DEHP was variable (o1–44%). Mean concentrations of DEHP and DEP in raw sludge were 30.2 and 1.60 mg/g dry wt, respectively. A mass balance for DEHP was calculated using data from field studies and estimates of sludge production at the works. The mass balance approach helped to provide information that could be used to improve the design and operation of sewage treatment works.