Technical Abstract: Methods for the evaluation of root rot disease in sugarbeet caused by pathogenic fungi historically have relied on visual assessment. In an initial attempt to develop complementary means for evaluating levels of root rot disease caused by A. cochlioides, antiserum was produced in rabbits that had been immunized with a cell-wall preparation of this organism. Specificity tests using enzyme-linked immunosorbant assay (ELISA) indicate that the antiserum is strongly reactive with both A. cochlioides and A. euteiches, but weakly with oomycetes non-pathogenic to sugarbeet, with filamentous fungi that infect sugarbeet, or with extracts prepared from healthy sugarbeet. A 1:2,000 dilution of the serum was sufficient to readily detect A. cochlioides in infected sugarbeet seedlings. Sugarbeet roots obtained from a piling station in Minnesota, USA that exhibited adult root rot symptoms characteristic of those caused by A. cochlioides tested negative for the presence of this pathogen. Use of the antiserum for the detection of A. cochlioides in sugarbeet infected in the field and in greenhouse screening programs for evaluating varietal resistance to this pathogen are discussed.