Attenuated strains of avian Eimeria parasites, generated by the selection of precocious lines through serial passaging in chicks, have been used widely as live vaccines. Detailed morphological transitions including their life cycle depending on the passages remain poorly understood. Here, we showed early development and acceleration of transitions in morphological forms of the asexual schizonts of E. tenella that had been attenuated for virulence by serial passaging. Our results may be helpful in understanding parasitism, facilitating further molecular analyses such as comparative genomic or transcriptomic tests.