Summary.Prometryne, linuron and chlorpropham at concentrations between 10 and 100 ppm had only temporary effects on the growth of three strains of bacillus sp. type mesentericus in liquid culture; growth of Pseudamonas phaseoli was not affected. The sensitivity of Ps. phaseoli lo pure antibiotics was only slightly affected by the herbicides, hence the effects of the herbicides on the antagonism between the two organisms which were observed appeared to be attributable to disturbance of the metabolism oi Bacillus sp., and varied with the strain employed.