SUMMARY : An organism with a new type of polar monotrichous flagella is described. The flagella have a very short wavelength which is very characteristic. The organism liquefies gelatin and oxidizes ethanol to acid. All other common differential tests were negative. The name Pseudornonas diminuta n.sp. is suggested. The curvature of bacterial flagella is quite constant and sufficiently charac- teristic to be of considerable taxonomic importance. The usual wavelength of the flagella of such polar monotrichous bacteria as Pseudomonas aeruginosa and Vibrio comma is in the neighbourhood of 2p. In a large collection of cultures of Gram-negative bacteria submitted to the authors for flagellar study by Robert Keller of the Museum of Natural History of Philadelphia were found three cultures which showed very unusual monotrichous flagella, having a wavelength of only slightly over 0.6~. The cultures originated from fresh-water streams. Morphologically and physiologically the three cultures were identical and one description will apply to all. MORPHOLOGY OF ORGANISM The organism is a small Gram-negative rod averaging 0-5 by 1.0~ in the usual peptone media. Capsule or spore formation was not demonstrable. It is actively motile with predominantly a single polar flagellum. A flagellum at each pole is fairly common in young cultures, and occasionally two flagella may be found on one pole. The flagella have two unusual characteristics: (1) Many of the individuals show the flagellum originating from the periphery of the pole rather than from the centre of the pole, the latter being the more usual origin with monotrichous bacteria. (2) The flagellar curvature is most unusual with an average wavelength of only 0.62,~~ and an average wave amplitude of 0.32,~. The standard deviation of the wavelengths from the mean, based on measurements of twenty-five flagella, is 0.12~ and the standard error of the mean wavelength is 0.024,~. In Fig. 1 is illustrated the flagellar morphology of the organism. The insert in the lower right corner shows the flagellar morphology of a typical strain of Pseudomonas aeruginosa. The wavelength of the flagella of the strain of Ps. aeruginosa pictured is most common with monotrichous types of bacteria and averages slightly over 2,~. This statement is based on the study of over 100 strains, representing many, but mainly unclassified, species. Among the named species of Pseudo- moms studied may be mentioned Ps. stutxeri, Ps. Puorescens, Ps. saccharo- phila, Ps. mucidolens, and the polar multitrichous Ps. marginalis. The flagella