A new peritrich species highly tolerant to ammonium and nitrite, Epistylis camprubii n. sp., was found adhered to the biofilm of two advanced wastewater treatment plants treating high ammonium-loaded wastewater in Rubí, Spain. Its morphology, oral infraciliature and phylogenetic position in the peritrich clade were studied. The new species is a vase-shaped peritrich, constricted below the peristomial lip, with an in vivo average length of 58.7 ± 10.1 μm, average width of 32.0 ± 5.4 μm, and a longitudinally striated, compact stalk that occasionally exhibits uneven thickness and rarely shows transverse segments. The peristomial disc is commonly rounded or pointed, and rarely umbilicated. The C-shaped macronucleus is located in the adoral half of the body, and the only contractile vacuole lies in the adoral third of the zooid. The molecular analysis of the 18s gene sequence clustered E. camprubii n. sp. together with the other Epistylis, with the exception of Epistylis galea. Key words: Peritrichia, wastewater treatment, ciliates, 18s rRNA, ammonium, nitrite.