ostmortem examinations of 49 red squirrels (Sciurus vulgaris) found dead on the Isle of Wight revealed the presence of a Hepatozoon species in 18 of them (37 per cent). The prevalence of infection was highest in subadult animals and no juveniles were infected. The prevalence was higher in the squirrels dying from natural causes (nine of 12) than in squirrels killed in road accidents (seven of 27). The weight of infection varied, and there were heavy infections in squirrels dying from toxoplasmosis and bacterial pneumonia. A PCR-based assay was used to identify the presence of Hepatozoon species DNA in the lungs, and immunoperoxidase staining was used to confirm the identity of schizonts observed in histological sections. The nucleotide base sequence of the PCR products indicated that the organism was a novel species closely related to, but distinct from, Hepatozoon erhardovae of bank voles (Clethrionomys glareolus).