Studies of mycopopulation of medicinal plant seeds till this time were not conducted systematically. Remarkable yield reduction, as a cause of presence of different pathogens requires devotion of especial care to this problem. From this reason, the aims of our contribution were as follows.

- Identification of the whole spectrum fungi present in seed samples;
- Determination of percents of abundance of these fungi;
- Testing of pathogenicity of the most harmful species determined.

Tested St. John's wort seed samples were collected at four localities, i.e. Pančevo, Svrljig, Kačarevo and Gorobilje, during the years 1998 and 1999.

Health status of seed samples was evaluated by the means of standard methods (ISTA).

This way, following fungi were determined: Fusarium oxysporum Schlecht., Fusarium solani (Mart), Fusarium proliferatum (Matsushima) Nireberg., Fusarim semitectum Berk & Rav., Verticillium spp., Alternaria spp., Acremoniella atra, Aspergillus flavus, Penicillium spp., and Mucor spp. Among samples investigated, the most abundant species recorded on the contaminated seeds was Alternariom spp., followed with fungi belonging to the genus Fusarium.

St. John's wort seeds are populated with a number of microorganisms, among which few of them could significantly decrease yield of herb.

Keywords: St. John's wort, seed, micro-flora.