A total of 21 species of Cyathostominae was found in the lumen of the large intestine of 86 mature horses of various ages and breeds killed in south-west England during 1972–1974. Cylicostephanus longibursatus, C. goldi, C. calicatus, Cyathostomum catinatum, C. coronatum and Cylicocylus nassatus were found in over 80% of the horses, while 12 of the remaining species were detected in less than 30%. Quantitative studies on 55 horses showed the adult cyathostome burdens to rise to a sharp peak (average over 400,000/horse) in April-June and a lower one in November-December. Parallel fluctuations occurred in the lumen populations of 4thstage larvae. The most numerous cyathostomes were generally those species with the highest prevalence. The five most plentiful (C. nassatus, Cyathostomum catinatum, Cylicostephanus longibursatus, C. minutus and C. goldi) comprised over 90% of the total number of adult cyathostomes recovered. The three regions of the large intestine had adult cyathostome faunas of different composition. That of the dorsal colon was dominated by C. longibursatus and C. goldi, the ventral colon by Cylicocyxlus nassatus and Cyathostomum catinatum, while the sparsely inhabited caecum accommodated several equally numerous species. Each species showed a characteristic site distribution, 11 occurring predominantly in the ventral colon, eight in the dorsal colon and two in the caecum. Seasonal changes in the site distributions of C. catinatum and Cylicocyclus nassatus and in the size of the cyathostome burdens are discussed in relation to the epidemiology of cyathostome infections.