Infection by Blastocystis sp. has been detected for the first time in pigs in Spain. Detection was carried out by the ethyl acetate-formalin concentration method and direct microscopy. Vacuolated cells of Blastocystis sp. were found in stool samples from 27 of 360 pigs examined (7.5%). The protozoan was present on nine of 17 pig farms studied (53%). Infected pigs ranged from 1 month old to adults, but the organism was most frequent in 1–2 month old (18.4%) and 2–6 month old pigs (15.4%). No correlation was found between Blastocystis sp. infection and occurrence of diarrhoea in pigs.