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A population of Paramecium aurelia , syngen 9, in Blackford Pond, Edinburgh, has been sampled over a period of 6 years and the types of immobilization antigen present identified. 2. Three antigen-determining alleles at the G locus and six alleles at the X locus were found. 3. No significant changes in relative numbers of the various alleles were apparent over the period studied, nor was there any difference between opposite ends of the pond. 4. An appreciable proportion of G-type heterozygotes was found, but the numbers were significantly less than would be expected by random mating alone. Autogamy is presumed to be responsible for this deficit. 5. Whether there is any natural selection in favour of heterozygotes over homozygotes in unknown. A possible method for obtaining information on this subject is discussed.
Pringle et al. (Mon,) studied this question.