In ciliate protists, sex involves the temporary joining of two cells of compatible mating type, followed by meiosis and exchange of gametic nuclei between conjugants. Reproduction is by asexual binary fission following conjugation. For the many ciliates with fixed multiple mating types, frequency-dependent sex-ratio theory predicts equal frequencies of mating types, if sex is common in nature. Here, we report that in natural populations of Tetrahymena thermophila sexually immature cells, indicative of recent conjugation, are found from spring through fall. In addition, the seven mating types occur in approximately equal frequencies, and these frequencies appear to be maintained by interaction between complex, multiple mat alleles and environmental conditions during conjugation. Such genotype-environment interaction determining mating type frequency is rare among ciliates.Sex is an extraordinarily successful eukaryotic invention usually associated with reproduction. However, in ciliates, reproduction is by binary fission, and sex is limited to the temporary conjugal union of two cells for purposes of recombination and rejuvenation-i.e., micronuclear exchange and macronuclear replacement. To recognize suitable partners, ciliates are differentiated into mating types, a kind of self-not-self discrimination system, as opposed to true sexes. Hurst and coworkers (1, 2) have suggested that by exchanging only gametic nuclei at conjugation, ciliates minimize conflict between cytoplasmic genomes and, therefore, are free to evolve multiple mating types to maximize the choice of sexual partners. Simple extension of frequency-dependent sex-ratio theory suggests that multiple but fixed mating types should be equally frequent in breeding populations, a suggestion supported by two theoretical studies. Orias and Rohlf (3) constructed a deterministic model for three alleles at a single locus and found that there is a stable equilibrium in which mating types are equally frequent. In a more general study, Iwasa and Sasaki (4) found that evolutionary consequences greatly depend upon mating kinetics and sex-determining mechanisms. When opportunities for finding suitable conjugal partners are temporally limited, the number of sexes-i.e., mating types-increases. There is, however, limited information on ciliate sex and mating types in natural populations (5, 6) with which to test these models.Ciliates typically have two types of nuclei: a germinal, diploid micronucleus capable of meiosis and mitosis and a somatic, compound macronucleus controlling the phenotype of the cell. At conjugation, haploid gametic nuclei meiotically derived from the micronuclei are reciprocally exchanged, and, following fertilization, new macronuclei are derived from mitotic products of zygotic micronuclei, while the old macronuclei are destroyed (7). Replacement of the macronucleus not only initiates a new life cycle that may include change in mating type but also is associated with rejuvenating (8) and heterotic effects (9, 10).To determine the inciden...