Triplet conjugation occurs frequently when three different mating types of T. thermophila are mixed under appropriate conditions. Genetic studies show that fertilization is usually tripolar, with each conjugant contributing a pronucleus to one mate and receiving a pronucleus from the other. Triplets are less common and less stable when only two mating types are mixed, and nuclear exchange is not always reciprocal. Some conjugants contribute a pronucleus but do not receive one in return. Others receive two pronuclei instead of one. Such triplets characteristically yield one haploid, one diploid and one triploid exconjugant, all of which are able to establish vigorous clones.