The cost of the seeds represents only 5% of the total cost of the onion production, consequently the consumers tend to disregard the essential genetic and sanitary quality of the seeds used. In contrast, the increase of epidemics of new onion diseases, in the onion production region of the State of Santa Catarina, South Brazil, clearly shows that seeds are, undoubly, efficient pathogen disseminators. In order to verify the sanitary quality of the seeds used by farmers, onion seed samples, obtained from seed lats commercialized in the state in the 1989, 1991, 1992, and 1993 growing seasons, were collected and analysed. Fungi of 16 different genero, among which manv species are phytopathogenic were identified. The fungus Colletotrichwn gloeosporioides f.sp. cepae, the causal agent of the onion foliar antracnose was detected in onion seed samples, indicating its seed-borne introduction into new areas. Other important pathogens detected in the seeds were: Fusarium oxysporum f.sp. cepae, Alternaria porri, Heterosporium allii-cepae, and Botrytis spp. The fungus Trichoderma spp., shich is used as a disease biological control agent, was also defected. Pathogenic and epidemiological aspects of the most important fungi detected, are discussed in the paper.