After the third prezygotic division during conjugation of Paramecium caudatum
, migratory and stationary pronuclei are produced. The migratory pronuclei remain in the paroral region tightly against the conjugating boundaries; while the stationary pronuclei are located beside the migratory pronuclei. To date, however, it is not clear what causes this close side-by-side localization between migratory and stationary pronuclei. In the current study, immunofluorescence staining with monoclonal antibody of anti-α tubulin indicated that "U" or "V" shaped spindles connected the migratory and stationary pronuclei during the third prezygotic division. This observation accounts for the close localization between these two types of pronuclei.