Quantitative methods for measuring population density of 'Verticillium dahliae' Kleb. in soil were investigated using the soil dilution plate technique. The population density of 'V. dahliae' decreased with increase in soil depth but 70% of the propagules occurred in the top 15 cm of soil. Estimated population densities declined by 50% after air-drying the soil for 1 week but did not change significantly thereafter up to 64 weeks. Population estimates decreased as the weight of soil and the volume of soil suspension deposited per plate increased. However, population estimates did not change between 2 and 64 weeks of incubation. Soil pectate tergitol agar was the most suitable medium based on size, accuracy and precision of population density estimates and ease of identification of 'V. dahliae' compared to 13 other published media. However, soil extract is not an essential ingredient and biotin may increase recovery of 'V. dahliae' from soil. Isolates of 'V. dahliae' collected from potato stems or soil in potato fields were characterised using vegetative compatibility groups (VCGs) and restriction fragment length polymorphisms (RFLPs). Of 139 isolates collected from Alliston, Ontario, 81% were assigned to VCG 4B, 18% to VCG 4A and 1% to the newly identified VCG 6. Based on RFLP studies, the population consists of one predominant clone, RFLP group I. There was no difference in the genetic diversity of 'V. dahliae' isolates collected from potato stems and soil. Specific RFLP fingerprints were associated with certain VCGs and the results could be used to develop an RFLP-based VCG diagnosis system. The effects of flaming or removing vines and fumigating soil on Verticillium wilt of potato were studied in a commercial field between 1993 and 1996 to investigate the possibility of reducing fumigant use. Annual flaming reduced the population density of 'V. dahliae' in soil, Verticillium wilt severity and was profitable. Flaming should be recommended as a long term strategy for controlling Verticillium wilt of potato. Soil fumigation should be used to reduce populations of 'V. dahliae' in highly infested fields and thereafter flaming should be used to prevent the build-up of inoculum in soil. Action threshold levels used as decision tools in employing disease management strategies partly depend on the method used to determine the population density of 'V. dahliae' and the population structure of the pathogen.