A developmental stage of a species of eimeriorin Apicomplexa was found heavily infecting the kidney of the European cormorant Phalacrocorax aristotelis aristotelis beached on the coast of Galicia after the oil spill from the ‘Prestige’. The immature condition of the parasite made a precise taxonomic identification impossible. The infection caused kidney damage as a result of pressure atrophy and mechanical displacement exerted by oocysts which occupied large areas of host tissue. A cellular host reaction was always apparent in the kidney of the infected cormorants. Although the oil effect is assumed to be responsible for the death of the cormorants, the impact of the coccidiosis should be monitored in the European shag populations.