AbstractParasitism ofTrichoderma harzianumon Cystosori ofPolymyxa betaeTransmission electron microscope investigations revealed that cystosori ofPolymyxa betaeKeskin are,in vitroconditions, parasitized and completely degraded byTrichoderma harzianumRifai. The hyperparasite showed a high ability in colonizing excised, surface‐sterilized lateral roots of sugar beet and in invading the cystosori present in them. Both the walls of the root cells and of the cysts are enzymatically perforated. However, in cysts penetration, mechanical forces are also involved. After penetration, cysts undergo a rapid degradation and finally they appear to be completely empty or to contain only small amounts of degradated cytoplasm.Cyst walls are also strongly altered and deformed and only two thin osmiophylic residual layers are detectabl