Sequences of cytological changes occurring in some bacteria following irradiation of X-ray have been described by many workers. It is known that, unless bacteria receive a considerable amount of irradiation, they continue to grow and become elongated, only their cell division being inbibited. In the present study, the author gave physiological aspects on the filamentous Salmonella typhi H901W elongated by culture following after X-ray irradiation (2000 r). 1. It was observed by Warburg's manometric technique that most of enzyme activities in the elongated cells were reduced. Especially, the induction of a adaptive enzyme for glucuronic acid could not be observed in the elongated cells. 2. In the post culture following after X-ray irradiation, protein and RNA increased as long as cell body elongates but DNA didn't increase. This result coincides with the finding that irradiated Salmonella typhi elongated without the increasing of number of nuclear apparatus. 3. In the studies on the agglutination reaction of the elongated cells, the author found that the agglutinability with antibody were reduced and moreover the isoelectric point moved slightly against to neutral pH.