AbstractA new recessive conjugation lethal mutation was found inTetrahymena thermophilawhich was namedmrafor macronuclear resorption arrest. Other events affected by themramutations are separation of pairs, DNA replication in the macronuclear anlagen, and resorption of one of the two micronuclei. In wild‐type crosses 50% of the pairs had separated by 12 hr after mixing two mating types and had completed resorption of the old macronucleus 1–2 hr later. In contrast mostmraconjugants did not separate even by 24 hr after mixing and the old relic (condensed) macronucleus was seen in over 90% of them.After addition of 10mM calcium to the conjugation medium, themraconjugants did separate but they still failed to complete resorption of the old macronucleus and to replicate macronuclear anlagen DNA in the exconjugants. The calcium induced separation of themraconjugants occurred later than the separation of control pairs.During normal conjugation cell separation occurs before the first expression of known macronuclear genes and prior to processing of the macro‐nuclear DNA. Therefore, themraphenotype infers that separation of conjugants requires a signal which is produced by the macronuclear anlagen at an unusually early time. © 1992 Wiley‐L