Objectives Rabbit coccidiosis is a highly contagious disease caused by protozoal parasite,among which Eimeria magna is one of the most prevalent species.To evaluate the expression of exogenous protein of a newly obtained transgenic line ofE.magna(E.mag E-R)which expresses enhanced yellow fluorescent protein(EYFP)and red fluorescent protein(RFP),the endogenous developmental stages were investigated.Methods Coccidia-free rabbits at the age of 35~40 days were inoculated with E.mag E-R oocysts according to the time of sampling and euthanasia were performed at 48,66,86,120,144,152 h post infection(h p.i.).The infective doses were as follows: 107 oocysts for rabbits 48~86 h p.i,105 for rabbits 86~144 h p.i.and 104 for rabbits 144~152 h p.i.Tissue samples taken from the ileum and jejunum were thoroughly washed with HBSS,and the mucosal layer were scraped and smears were visualized using a confocal laser scanning microscope(SP5 Leica,Germany)for detection of EYFP-expressing parasites.Results Considering the EYFP was mainly expressed in the parasite nuclei,uninucleate(type A)and multinucleate(type B)merozoites in each schizogony were easily distinguished.Four generations of meronts,as well as gametes,gametocytes and immature oocysts were observed at time point of 48,66,86,120,144,152 h p.i,respectively.Our results showed that EYFP of transgenic E.mag was continuously expressed throughout the entire life cycle,although RFP was not observed except in the invasive sporozoites.Conclusion In this study,the endogenous developmental stages of a transgenic line of E.magna was investigated.Confirmation can be made that the exogenous protein of the transgenic line was expressed throughout the life cycle.This study provided the basic foundation of the study of transgenic rabbit coccidia.