A study was made of certain factors affecting the use of a phage–plaque-count technique for detecting in bean seed the organism causing common bacterial blight,Xanthomonas phaseoli(Erw. Smith) Dowson. It was found that overgrowth of the fluid test cultures with contaminating bacteria from the seed or from soil did not interfere appreciably with phage adsorption or multiplication in the presence of host cells. Such samples required centrifugation prior to testing in order that plaques might be readily discernible on agar plates. The phage was adsorbed specifically only by cells ofX.phaseoli. Tests with a variety of media showed that nutrient broth was adequate for the purposes of the phage–plaque-count technique. An incubation time of at least 24 hr. for phage and sample was shown to be desirable. Out of 75 samples tested in quadruplicate, 50% were found to be infected with the pathogen in varying degrees. Several samples gave positive reactions by means of this plaque-count method under conditions which were quite unfavorable for isolation of the pathogen by the usual methods.One-step growth studies withX.phaseoli-phage indicated a low order of adsorption, a latent period of 30 to 45 min., a rise period of approximately 30 min., and a burst size of 40 for stationary cultures and 30 for shake cultures. Electron microscopy ofX.phaseoli-phage preparations revealed a particle with a dense head 60 mμ in diameter and a tail 170 to 190 mμ long and 10 to 15 mμ wide.