A pythiaceous phycomycete was isolated from diseased roots of rhododendron in New York. It was characterized by fast growth, preference for high temperature for growth, ovoid to elongate, non-papillate, internally proliferating sporangia and large spherical, pigmented chlamydospores, but failed to produce sex organs in single or dual cultures. It was identified as Pythium dimorphum Hendrix & Campbell. This appears to be the second report of its occurrence since its first isolation from lolly pine roots in Louisiana in 1971.