The aim of the studies was determination of Trichoderma effect on primordia formation of crossbred cultures of Agaricus bisporus. Isolates of two Trichoderma species originated from domestic mushroom farms, i.e. T. aggressivum f. europeaum and T. hamatum were used. The assessment of the primordia formation was carried out in glass cylinders of 600 cm3 volume and 10 cm diameter. It was found that both Trichoderma species influenced significantly the number of primordia formed by strains and crossbred cultures of A. bisporus. All the T. aggressivum f. europeaum isolates, i.e. T. 361, CBS115901 and T. agg. 29/15 drastically limited the primordia formation by Agaricus strains and crossbred cultures. It was demonstrated in the case of T. hamatum that isolate T. ham. 7/33 decreased the number of primordia to a much greater extent than the other two isolates T. ham.3/74 and T. ham. 9/11.