The purpose of this study was to relate soil inoculum density of Verticillium dahliae to systemic stem colonization by the fungus over time. Within four 64-ha commercial potato fields 100 small plots were located on a grid covering most of each field. The soil inoculum density of V. dahliae was determined for each plot. Percent potato stems colonized by V. dahliae, Colletotrichum coccodes, and Erwinia carotovora and root colonization by Pratylenchus penetrans was determined at four times during the season by destructively sampling plants within individual plots on the grid. A linear logit model adequately fitted soil inoculum density of V. dahliae vs. stem colonization data albeit with a large random error term for the model. Data were grouped into inoculum level classes according to soil inoculum density as follows: 0 (fungus not detected), 1-5, 6-10, 11-25, and >25 ppg. The proportion of stems colonized increased with increasing soil inoculum level of V. dahliae at any given sampling time. This was also true when stem colonization by V. dahliae was measured by percentage of vascular bundles colonized. A stem had a greater probability of being infected at its base than at its top for each sampling time and for any given soil inoculum level. The proportion of stems infected by V. dahliae increased over time. The rate of disease progress was similar regardless of inoculum density except when no V. dahliae was detected in the soil, in which case the rate was close to zero. Incidence of stem infection by soft-rot E. carotovora increased over time to nearly 100% by the last sampling time. There was no evidence that E. carotovora influenced the dose-response relationship between V. dahliae and proportion of stems infected. Similarly, there was no evidence that C. coccodes influenced the relationship between soil inoculum levels of V. dahliae and colonization of potato stems. Root infection by P. penetrans significantly increased the probability of stem infection by V. dahliae in some cases