Sarcocystis was detected in oesophageal samples from 245 (62 per cent) of 394 horses and ponies killed at a Cheshire abattoir between February and August 1981. Prevalence of infection was closely related to age, increasing from 28.6 per cent of animals up to two years old to 88.9 per cent of those over eight years old. There were no significant regional differences in prevalence between horses from north west England, Yorkshire or South Wales. Significantly more female horses were infected (69.7 per cent) compared with males (56.2 per cent). Gross examination methods detected fewer than 55 per cent of all infections compared with a tissue digest method. Sarcocysts ranged from 1 to 15 mm in length (mean 5.5 mm) and were up to 0.5 mm wide. Cyst wall morphology and bradyzoite dimensions suggest that the species involved is Sarcocystis bertrami (syn equicanis).