Serious losses caused by root rot of sugar beet were observed in Poland in 2001 and 2002. The disease occurred in most of regions of sugar beet cropping from June to the end of growing season. Very high losses of yield were observed on many fields especially on cultivar Lolita in 2001. The aim of this work was to detect the casual agent of root rot (2001) and to compare susceptibility of sugar beet cultivars to this disease (2002–2003). The laboratory tests (2001) showed that most of destroyed roots were infected by Aphanomyces cochlioides Drechsler. This species is very well known as a pathogen of sugar beet seedling damping-off in Poland. The fungus Rhizoctonia solani Kühn was found only in 12% of rotten roots collected in south-eastern part of Poland. In field trails the highest number of infected roots and the highest infection inde xwere found for cultivar Arthur (2002–2003). That cultivar was significantly more susceptible to root rot than the rest of tested cultivars