Bacterial variation has been studied mainly in vitro, few investigations having been made on the effect of host passage on the pathogen. The common belief is that virulence is maintained or enhanced by passage through a host. Outstanding as a critical analysis of this problem is the work of Wellhausen (19), who determined the effect of repeated passage of Phytomonas stewartii (E. F. S.) Bergey et al. through resistant and susteptible lines of maize and through certain other grasses. Successive passage through susceptible maize decreased virulence, while successive passage through resistent maize increased virulence. This investigation has been devised to check and extend Wellhausen's observations on the effect of the host upon bacterial virulence and to determine the factors that operate to bring about such changes. The same organism, Phytomonas stewartii, has been used.

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