A mycological survey was carried out at the Northern part of Tunisia on freshly harvested durum wheat in 2007, as wheat is the basic staple food for the Tunisian population. A total of 65 samples was analysed for their contamination with pathogenic and saprophytic fungal species and isolation frequencies and relative densities of the fungal species were recorded. Our results showed that Alternaria species were observed in all the prospected samples and Fusarium spp. were present in about 70% of fields whereas the incidence of the genus Penicillium was low. In the light of the mycological analysis, we evaluated the cytotoxicity of the possible occurring mycotoxins in such crops using the MTT bioassay. For this purpose, four mycotoxins — Deoxynivalenol (DON), Alternariol (AOH), Ochratoxin A (OTA) and Citrinin (CTN) — were screened for their toxicity in human colon carcinoma cell line HT 29. The viability assay showed significant variation in IC₅₀ concentrations between the tested mycotoxins.