Technical Abstract: Bacterial Leaf Spot, caused by Xanthomonas campestris pv. vitians (Xcv), is a widespread and economically important disease of lettuce. Cultivars with resistance to this pathogen have been identified, however mechanisms for resistance in this pathosystem have not been investigated. We hypothesize that susceptible and resistant cultivars differ for the populations of the pathogen that they support on leaf surfaces. A rifampicin resistant strain of Xcv was selected and compared to the wildtype and was used to inoculate susceptible (Vista Verde) and resistant (Little Gem) cultivars and four cultivars with intermediate in resistance. Five-week-old seedlings were inoculated with 1 x 104 CFU/ml of Xcv until runoff. Eight times over 28 days, leaf tissue (0.8 cm2) from six replicates of each cultivar was macerated and serial dilutions were spread on nutrient agar amended with rifampicin. Colonies were counted after incubation at 27'C for four days. Populations were variable on all cultivars but in general Vista Verde had one of the highest populations seven days after inoculation, while Little Gem had the lowest populations overall. Statistical analysis showed that on both 21 and 28 days after inoculation there were significant correlations between disease ratings and leaf populations. The results of this first experiment indicated that resistant and susceptible cultivars differ in their influence on the population levels of Xcv, with susceptible cultivars supporting higher pathogen populations. We are currently repeating the experiment. These results indicate that factors influencing pathogen populations could be productive targets for research on resistance mechanisms.