The Guelph strain of Eimeria adenoeides was obtained from a commercial turkey flock in Ontario, Canada, in 1985. Single oocyst derived lines of E. adenoeides were propagated, and one of them used to re-describe biological and morphological features of E. adenoeides in the turkey. Oocysts of this strain are within the lower size ranges in the original species description reported by Moore and Brown (1951); oocysts of the Guelph strain averaged 18.7 ± 1.4 μm (16.7-22.5) by 14.3 ± 0.9 μm (13-16.2, n = 30) with a shape index (SI) of 1.3 ± 0.1. It is possible that the original species description was based, at least in part, on a mixed culture of two or more Eimeria species. Immature first-generation meronts of E. adenoeides Guelph strain were observed histologically at 32 h post-infection in the ileum and cecal neck. Early studies reported only two asexual generations suggested that first asexual cycle observed at 32 h post-infection was overlooked. In the present study, three asexual generations were observed before the start of gametogony. The Guelph strain is also characterized by a prepatent period of 112 h. The Guelph strain of E. adenoeides is a highly pathogenic coccidium that forms classic cecal lesions, including prominent caseous cecal cores, during moderate to severe infections. The maximum output of oocysts (1.77 × 10(7) per bird) was obtained from birds inoculated with 1 × 10(3) oocysts; maximum fecundity (1.55 × 10(5) oocyst shed per oocyst inoculated) was obtained with an inoculation of 1 × 10(2) oocysts, but fecundity dropped dramatically as the inoculation dose increased. To promote stability of the E. adenoeides species concept, neotype specimens (a parahapantotype slides series and phototype) have been designated and deposited for future reference.