A bioassay was developed to determine the effective inoculum density of A. euteiches in naturally infested field soil. Infested soil was mixed with steamed soil to produce a series of dilution levels. Each dilution level was divided into aliquots, and each aliquot was tested for the presence of the pathogen using a host bioassay under optimal conditions for infection. The proportion of aliquots infested at each dilution level was used to calculate the infective inoculum density of A. euteiches in the original soil by means of the most probable number (MPN) method. A computer program was written to calculate the MPN for any series of up to 5 dilutions and any number of aliquots per dilution. In addition, the program calculates confidence limits for the MPN and the statistical acceptability of the MPN. MPN estimates of infective inoculum density in field soils were consistent with the results of a standard root rot potential test and were correlated with epidemiologic and yield measurements from field plots of diseased peas.