Unsporulated oocysts were detected in the feces of an Arabian red fox (Vulpes vulpes arabica) between 6 and 8 days after it had been fed meat from Arabian mountain gazelles (Gazella gazella) known to contain sarcocysts. No oocysts were discovered in the feces of other experimental cubs, although sporocysts of Sarcocystis spp. were passed subsequently by all cubs that were fed gazelle meat, including those fed with reem (G. subgutturosa marica). The oocysts sporulated in 3 days at room temperature (25 +/- 2 C); they were 10.9 +/- 1.4 x 10.1 +/- 1.3 microm, with 2 sporocysts measuring 6.0 +/- 0.6 x 4.7 +/- 0.8 microm, each with 4 sporozoites. Sporulated oocysts were identified as those of Hammondia heydorni using molecular and standard morphometric techniques. Sequence differences between 2 fox and 3 dog isolates of H. heydorni were detected and allowed differentiation between the 2 populations of the organism. The involvement of Neospora caninum was excluded using molecular methods. The Arabian red fox and the Arabian mountain gazelle in Saudi Arabia are new, definitive and intermediate hosts for H. heydorni.