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 Xcv have been identified, but mechanisms for resistance in this pathosystem have not been investigated. We hypothesized that susceptible and resistant cultivars differ in terms of populations of Xcv that they support on leaves. A rifampicin resistant strain of Xcv was inoculated onto seedlings of susceptible (Vista Verde) and resistant (Little Gem) cultivars and four cultivars intermediate in resistance. Bacterial populations were estimated by spreading dilutions on media containing rifampicin. In general, susceptible cultivars, including Vista Verde, supported the highest populations of Xcv, while Little Gem had the lowest populations by 14 days after inoculation. In initial experiments, in which the presence or absence of lesions was not considered in sampling, high within-treatment variability was noted; subsequently, we determined that lesions on Vista Verde had higher populations than non-symptomatic tissue. As a result, in subsequent experiments both the most severe lesions (once symptoms developed) and non-symptomatic tissue were sampled. Preliminary data indicated that an increase in populations of Xcv on Vista Verde coincided with lesion occurrence. These results indicated that factors influencing Xcv populations could be productive targets for research on resistance mechanisms.