Modifications were made to Specht's TB-CEN medium that increased its selectivity for isolating Thielaviopsis basicola from cotton field soils in the San Joaquin Valley of California. This allowed the quantitative determination of pathogen populations in naturally infested cotton field soils. In an extensive survey of Kings County cotton fields the pathogen was detected in 88% of the fields surveyed. Black root rot was detected in 66% of the surveyed fields. Disease severity was positively correlated with inoculum density, and pathogen populations were positively correlated with the number of years in cotton. T. basicola was found less frequently and in lower inoculum densities in fields in which crop rotation or summer flooding had been practiced, when compared with fields planted continuously to cotton.