Abstract: To study the somatic function of the micronucleus of the ciliated protozoan Pseudokeronopsis monilata,amicronucleate cells were generated by removing the micronuclei shortly before binary fusion.Most of the amicronucleate cells derived from regeneration were crippled in morphology,characterized by the defective adoral zone of membranelles.At the same time,the number and the size of macronuclei in some amicronucleates become unstable.These suggest that the micronuclei play an important role for maintaining the stable structure of the cell and the macronuclei.