The early development of Eimeria papillata (Apicomplexa) was studied in vitro using light and electron microscopy. The first developmental stages observed were elongated, sporozoite-shaped schizonts with two nuclei and two refractile bodies which appeared 17 hours p.i.; the schizonts retained this shape until the 8-nuclei-stage. At 23 hours p.i. they had changed into a spherical shape, the refractile bodies had multiplied and the development of merozoites was observed. Each merozoite of the first generation contained two refractile bodies. An improved method for in vitro cultivation of Eimeria in polarized host cells and subsequent fine structural investigation is described © 1995, Gustav Fischer Verlag Jena. All rights reserved.