Objective To illustrate the ultrastructure of the spermatozoon of Raillietina cesticllus.

Method Transmission electron microscope was used to observe the morphorlogy and structure of the spermatozoon of R. cesticllus parasitized in chicken intestine.

Result The spermatozoon of R. cesticllus is filiform, tapers at both ends, has crested-like body and lacks the mitochondria. It has axonemes with the "9+1" pattern. Based on longitudinal sections of R. cesticllus spermatozoon, five regions could be distinguished. RegionⅠcorresponds to the anterior extremity of the spermatozoon, having a crested-like body. RegionⅡ is characterized by the presence of cortical microtubules, a periaxonemal sheath and intracytoplasmic walls. Region Ⅲ has plenty of electro-dense granules. Region Ⅳ is characterized by the presence of the nucleus. RegionⅤcorresponds to the posterior end of the spermatozoon, having free flagellums. The spermatozoons of R. cesticllus and R. echinobothrida each has one crested-like body, while the spermatozoons of all other six reported davaineid cestode each has two crested-like bodies. The rotation angle of the crested-like body of R. cesticllus is 40°to 55°, other than 35° for R. echinobothrida.

Concluison The number and rotation angle of the crested-like body of the spermatozoon head can descriminate R. cesticllus from other seven reported Davaineidae cestode.