A strain of Pseudomonas fluorescein which synthesizes large quantities of an inducible extracellular pectate lyase proved to act as a virulent soft rot organism on potatoes, whereas a parent strain that synthesized no lyase showed little pathogenicity. The procedure for inducing lyase synthesis involves culture of P. fluorescens for at least five generations on a combined carbon source of pectin plus a factor extracted from acetone powders of potatoes. The extracted factor is dialyzable and heat-labile but can be sterilized by Millipore filtration. When soil was inoculated into the induction medium, microorganisms present synthesized pectate lyase.Less than half of the single colony isolates obtained by plating out induced cultures of P. fluorescens synthesized pectate lyase when reinoculated into induction medium. Serial reisolation and reinoculation of induced cultures failed to increase the proportion of induced cells. This failure suggests that inducibility is lost during growth of cells on induction medium and (or) on solid agar used for plating. Under similar conditions, cultures of Erwinia carotovora contained induced cells only. The apparent loss of inducibility that occurs in P. fluorescens may account for its generally saprophytic behavior in nature.