Polyclonal antibodies were prepared against crude cell wall fractions of a Pythium isolate with filamentous non-inflated sporangia, associated with root rot of tomato in soilless cultures. In ELISA tests, the antiserum showed sensitivity and specificity against original antigens and those from closely related isolates with filamentous sporangia (such as Pythium aquatile and Pythium coloratum). The antiserum reacted weakly with mycelial preparations of several fungi commonly found on tomato roots, such as Fusarium spp. or Penicillium spp. An immunocytochemical staining test was developed in order to study the colonization by filamentous non-inflated sporangial isolates of Pythium in the presence of other fungi in soilless cultures.