Gene action in Paramecium aiurelia has been investigated (Sonneborn, '43a) most fully in the case of the action of the gene involved in the determination of the difference between "killers" and "sensitives." One race of this species makes the fluid in which it lives poisonous to nearly all other races of Paramecium. This race is thus a "killer." Killers are invariably resistant to their own poison. Races affected by this poison may be called either "sensitives" or "non-killers," for sensitives are never killers. Likewise, non-killers are never resistant. The killer character is determined by the combined presence of a dominant gene, K, and a cytoplasmic factor, kappa. Killers always have kappa; without kappa, clones are always sensitive non-killers. Killer clones also always have, in addition to kappa, gene K; but non-killers, lacking kappa, may have either K or its recessive allele, k. Thus, neither K nor k can initiate the production of kappa. Nevertheless, there is some relation between these genes and kappa, for clones containing kappa always have K and are never homozygous for k. The role of the genes is shown by observations on homozygous recessives (kk) produced by the self-fertilization of heterozygous killers (Kk plus kappa). In these recessives kappa is retained for a few fissions, during which the cells remain killers; hence, cells are killers when kappa is in the cytoplasm and K is absent from the new nuclei (though still present in disintegrating parts of the old nucleus). However, after a few fissions, kappa disappears and the clone becomes and remains permanently non-killer. Hence, kappa is not independently self-multiplying; it depends upon gene K for its maintenance and increase. The role of the genes is further shown by introducing kappa into non-killers. If the non-killer has the genotype kk, the resulting clone is still a non-killer; but if the non-killer has the genotype KK or Kk, introduction of kappa results in its maintenance and increase, yielding a clone of killers. Hence in relation to gene K, kappa acts something like a primer in a pump: some kappa is put in and more comes out. K seems to be like a pump that will not work without being primed. The action of gene K in controlling increase of kappa is thus dependent upon a cytoplasmic primer, kappa itself. The work just summarized suggests that an understanding of the action of gene K might be acquired by a detailed investigation of its relations to kappa. An investigation of these relations is reported in a paper now in press (Sonneborn,