Strains ofBacillus, PseudomonasandChromobacteriumisolated from dead leaves of lettuce seedlings overwintering in the open have been proved to be antagonistic in various degrees toBotrytis cinereaon nutrient agar, on wounded detached lettuce leaves and on wounded leaves of lettuce seedlings at temperatures ranging from 4 to 25° C. under conditions of high moisture and humidity. A strain ofMicrococcuswas completely non‐inhibitive. Antagonism by the strains ofBacillusand byBacterium coliwas evident only at temperatures above 15–20° C. while the other strains were effective also at temperatures as low as 4° C. Bacterial growth on agar and on dead lettuce tissue usually raised the pH from 6.1 to 7.8–8.4 at which level growth ofBotrytis cinereais almost negligible and the activity of any pectinase it produces is very much reduced. The antagonism of the bacteria investigated, however, was largely due to antibiotic substances quite independently of the effect of the high pH. Some bacteria which did not give rise to wide inhibition zones were nevertheless strongly antagonistic in the immediate neighbourhood of the colony.B. cinereaon nutrient agar and on lettuce leaves produced a strongly acid reaction. This was capable of neutralizing the alkaline reaction due to most bacterial activity only if the latter was not already well established. The mixed bacterial flora of all dead lettuce leaves tested was potentially inhibitory toB. cinereato a level as high as or higher than that shown by the majority of the test strains in pure culture. Many strains of bacteria cause lysis of young activeBotrytismycelium and germ tubes. In association with many bacteria,B. cinereahas exhibited features such as stimulation or depression of sporulation, continuation of aerial growth with inhibition of lateral spread of mycelium, vesicular distortion of hyphae and germination of spores while still attached to the conidiophores. Some spores of the test strain ofB. cinereaused gave rise to new strains more resistant to antagonism than the