Coomassie Brilliant Blue Agar was used to quantify the frequency of the A-layer phenotype in different isolates of Aeromonas salmonicida. Hydrophilic, non-clumping isolates of A. salmonicida consisted predominantly of the A-layer minus phenotype. These bacteria were avirulent by intraperitoneal injection into susceptible brook trout (Salvelinus fontinalis) and could not be reisolated from infected fish. By contrast, hydrophobic, clumping isolates were predominantly of the A-layer positive phenotype, highly virulent in brook trout, and easily recovered from dead or moribund fish. A-layer positive and negative clones of A. salmonicida were derived by plating bacteria on Coomassie Blue Agar. The plating showed clearly that Coomassie Blue Agar could be used as a highly selective in vitro screening method to reclaim the virulence of certain isolates of A. salmonicida having a relatively low percentage of A-layer positive phenotypes.