Fuscous blight (Xanthomonas phaseolivar.fuscans), a new disease of field beans in Canada, was found to be present in a high proportion of bean seed samples from southwestern Ontario. Although symptoms of the disease are similar to those of common blight (Xanthomonas phaseoli), a greater intensity in color is apparent on infected seed and more hypertrophy, especially of nodal tissues, is caused by fuscous blight than by common blight. In epidemiological studies in the field, it was found that seed infection as low as 0.5% could cause a serious epiphytotic in the resulting crop. This points out the necessity of a zero tolerance for fuscous blight in valuable seed stocks. Furthermore the rapid phage plaque method can be used to determine zero tolerance. Garden varieties were generally more susceptible to the pathogen than field varieties. Great Northern showed the highest degree of resistance to the disease of all the varieties tested.An agar plating technique was developed whereby the organism can be isolated and identified in infected bean seed by the production of a brown diffusible pigment on nutrient agar. A bacteriophage spot test is also described which can also be used to identify the pathogen from infected bean seed. By means of phage typing, the relationship ofX. phaseolivar.fuscans, X.phaseoli, and intermediate strains is shown.