The probability of spreading cysts of Globodera rostochiensis on farming equipment and potato tubers was investigated in naturally infested field plots. The number of cysts recovered from soil that adhered to equipment differed significantly between different pieces of equipment. These differences were related to initial nematode density and, in most cases, to the volume of soil that adhered to the equipment. At an initial density of 0.04 egg/cm(3) of soil, significantly more cysts were recovered from a potato digger than from a potato hiller, cultivator, or plow. At an initial density of 0.90 egg/cm(3) of soil, significantly more cysts were recovered from the plow than from the other equipment. Although the population density was 22 times greater, only 10 times more cysts adhered 3 to equipment used in soil with a density of 0.90 egg/cm(3) of soil than when used in soil infested at 0.04 egg/cm(3). The number of potato tuber samples (4.5 kg) that contained cysts with viable eggs was positively correlated with the initial densities of G. rostochiensis in soil in which they were produced. The percentage of tuber samples with cysts containing viable eggs was 10-12% for tubers harvested from soil with densities less than 1 egg/cm(3) and 30-76% for tubers harvested from soil with densities greater than 4 eggs/cm(3) of soil.