Foal heat diarrhoea is a common diarrhoic syndrome in foals; it often develops during the period when the mare's first post-partum estrus is expected, hence the name. Seventy-five to 80% of foals display this syndrome between six to 10-14 days of age; faeces are soft to watery, and afflicted foals appear healthy and have a normal appetite. Foal heat diarrhoea is usually self-limiting in three to four days and rarely needs treatment, but this seemingly innocuous condition may predispose some foals to more severe and prolonged diarrhoea (Masri et al., 1986). The aim of the present study was to evaluate the role of Cryptosporidium parvum as a cofactor in the etiology of heat diarrhoea. In all the subjects Salmonella enterica was not isolated. Rilevant differences were not detected between E. coli and Cl. perfringens bacterial charges of foals with and without diarrhoea. E. coli and Cl. perfringens bacterial charges of mares were within normal values. All 30 examined foals seemed to have adequate passive transfer of maternal immunity as determined by the zinc sulfate turbidity test. All the foals were negative for E. leuckarti, S. westeri and Giardia. Twenty-four foals showed foal heat diarrhoea at 7-10 days of age. Twelve foals resulted infected by Cryptosporidium; 41.6% of diarrhoic foals and 33.3% of healthy foals had Cryptrosporidium in their feces. All mares were negative for E. leuckarti, S. westeri, G. duodenalis and C. parvum. Oocysts of Cryptosporidium were found in 33.3% of foals without diarrhoea. The number of foals that presented contemporary a high number of oocystis in faecal samples and foal heat diarrhoea could suggest a possible role of Cryptosporidium as a co-factor in the etiology of foal heat diarrhoea.