Strain D of species (syngen) 1, Tetrahymena pyriformis, differs from other inbred strains in its manifestation of certain abnormal patterns of adoral membranelles. Instead of the usual three membranelles some cells have a greater number, most frequently 4 or 5, but occasionally up to 7. The extra membranelles, or even all membranelles of any given set, are shorter than M-1 and M-2 of the normal pattern. In other cases, the only alteration observed is a change in the relative lengths of the three membranelles. The frequency of abnormal cells varies from about 5% to 15% during exponential growth to over 50% after prolonged stationary culture. The genetic basis for the abnormality is shown to be due to a single recessive gene which segregates normally in various crosses and which manifests vegetative assortment as do most allelic variants in species 1.