A hypodiploid strain of Tetrahymena thermophila has been obtained that shows arrest at the stage of condensed nuclei, corresponding to metaphase I of normal conjugants and induced arrest at meiotic metaphase I (i.e. at the stage of condensed, bivalent chromosomes) in its wt partner mate. The metaphase I arrested conjugants retained their old macronuclei and most of them underwent cell fusion, instead of separation of exconjugants. The doublets were viable and cortically integrated. When the arrest inducing strain was crossed to the haploid tester strain, the haploid micronuclei were arrested in the meiotic metaphase I as the diploid ones had been; the monovalent, chromosomes were condensed, the arms of sister chromatids were not separated, and they were not segregated. Separation of the arms of sister chromatids and disjunction of bivalent chromosomes were not prerequisite for the formation of microtubular spindles in those cells that were arrested in meiotic metaphase I. After re–feeding, the doublet cells resumed cell divisions, segregating two macronuclei and micronuclei at random. One macronucleus was derived from the arrest inducing strain and the other from the tester strain. Heterokaryon strains with macronuclei derived from the parental arrest inducing strain and with the micronucleus derived from the parental wt tester strain were obtained. Surprisingly, these heterokaryons did not induce meiotic arrest. Thus, the arrest in the meiotic metaphase I was induced by the micronucleus and not by the macronucleus of the arrest inducing strain.