SummaryI. By means of a special method of staining, developed by the writer, the following observations onBacterium tumefaciensare made. First, a very marked variation in size in young cultures, varying from 0.6 to 3.6μin length, and 0.3 to 1.0μin width. Second, the presence of many club-shaped rods in young cultures, which appear heavily stained and homogeneous by ordinary staining methods and which are found to consist of plump rods with extensions or buds from one end, by the special staining method. These buds comprise the narrow portions of the clubs and are found to possess a heavily staining interior body which is found extending at first into the main body of the rod. The process is stated to be comparable to the budding of yeasts. Third, the presence of heavily staining, bipolar bodies in ordinary rods. Evidence is presented to show that these rods are not spores and that no spores are to be expected in this organism. Fourth, a central body found in rods possessing one polar body, the central body being in the form of a band or of two particles opposite each other, suggesting a mitotic figure. Other evidence is presented indicating a nuclear phenomenon prior to cell division.II. Using a Berkefeld V filter in preliminary experiments, the filtrate is found to contain small rods, which when transferred give rise to characteristic growths ofB. tumefaciens.Checked with a culture ofBacillus amylovorus, a small rod former but possessing no tiny buds or minute rods, the filtrate remained sterile. Attention is called to the bud-like processes described by Barnard as occurring in the virus of various tumors and comparison is made with this and the bud-like extensions on rods ofB. tumefaciens.