Seed endophytes are potential bioagents for plant protection and growth promoters. The question of the specifics of their isolation in cultural environments is not clear. The purpose of this study was to establish the nature of the interaction of endophytic fungi of wheat seeds with different levels of aggressiveness and presence in the mycobiota. Dual cultivation was carried out at PGA, comparing with single fungal cultivation. The mutual influence of fungi during joint cultivation was established. Alternaria arborescens, which dominated in the mycobiota of wheat seeds from northeastern Ukraine, suppressed the development of only Penicillium. Nigrospora oryzae, Bipolaris sorokiniana, and Phoma developed faster than A. arborescens. Fusarium poae, and F. sporotrichioides competed for agar medium with N. oryzae. Known bioagents from wheat seeds showed unexpectedly low results. Trichothecium roseum formed a rejection zone during co-cultivation with F. graminearum. Trichoderma sp. Max18 (resistant to fludioxonil) on the 7th day inhibited the development of Penicillium, F. graminearum, and A. arborescens by 55, 48 and 26%, respectively. N. oryzae developed faster than the mycoparasitic fungus, but the latter began to parasitize it only from the 13th day.