Bacteria have long been employed for the study of various aspects of genetics, particularly mutations. Their rapid reproductive rate, accompanied by a pattern of mutations that appear spontaneously, allows investigations of several generations in a relatively short period of time. Methods of inducing mutations in bacteria artificially have been studied for many years, and one of the most frequently used groups of artificial mutagenic agents on bacteria has been radiation from various sources. Rubin ( 1 ) has established standard procedures for carrying out studies on the effects of radioisotopes on bacteria. His work dealt with isotopes of high activity, and necessary precautions for prevention of harm from radiation to personnel, and prevention of contamination to organisms in the immediate vicinity are described. Since our work dealt with low-activity isotopes, these precautions proved unnecessary. Alder and Hardigree (2) have investigated the sensitivity of Escherischia coli to radiation from an ultraviolet source. Their work isolated loci in the gene which controlled this sensitivity. Clark and Goring (3) found that radioactive phosphorus exerts a lethal effect on certain bacterial cells, while allowing other cells to pursue a normal growth pattern. One of the organisms studied was E. coli, and the activity range was in the order of 1.0-5.0 millicuries. We used radioactive phosphorus in this study, and E. coli was one of the organisms investigated, but the level of activity in our studies was no more than 100,000 micromicrocuries. Rubin (4) also investigated radiationinduced mutations in E. coli, using a deep-therapy unit designed for medical X-ray work, producing 5,000 roentgens per hour. His study set up standards for the quantitative estimation of these effects. The purpose of this study has been to investigate the mutagenic effects of low dosages of radioactive phosphorus (p32) on six different bacteria. The organisms were selected to include bacteria with different properties visible microscopically, as well as those which would produce colonies with easilyseen pigmentation and form. In this way, any changes produced by the introduction of the p32 could be observed either microscopically, macroscopically, or both. The organisms used were (1) Escherischia coli, a Gram-negative short rod, which produces cream-colored spherical colonies; (2) Alcaligenes foecalis, a Gram-negative rod, whose colonies are pale yellow; (3) Serratia marcescens, a Gram-negative short rod, with brilliant red colonies; (4) Bacillus cereus, a Gram-positive, spore-forming, long rod, which forms milky-white colonies; (5) Sarcinia lutea, which is a Gram-positive coccus with bright yellow colony formations; and (6) Micrococcus roseus, another Gram-positive coccus, with tiny rose-colored colonies. In addition to differences in appearance, the bacteria were investigated in light of their physiological reactions, but this phase of the project was used only for purposes of acquaintance with the organisms, and was not involved in the final results.