Effect of some Chemical and Biological Fungicides on Mycelial Growth and Disease Severity of Fusarium oxysporum f.sp. radicis-lycopersici. Fusarium is among the most aggressive telluric fungi causing wilt and root rots in several vegetable crops. A new disease caused by Fusarium oxysporum f. sp. radicis-lycopersici was recorded in southern Tunisia in the "Cinquième Saison" farm, situated in Hammet Gabès during 2000-2001 crop season. It caused death of up to 90% of tomato plants in some greenhouses. In the present research, the effect of 3 chemical and 4 biological fungicides was tested in vitro on mycelial growth and in vivo on disease severity of F. oxysporum f. sp. radicis-lycopersici. Hymexazol, benomyl and manebe were used as chemical fungicides whereas four biological products, i.e. two based on Trichoderma harzianum, one on Bacillus subtilis and another one on Bacillus thuringiensis were also tested. Among the chemical fungicides which were used, hymexazol and benomyl were the most effective in vitro and inhibited mycelial growth up to 80%. However, in vivo assays showed that only hymexazol was effective with a reduction in disease incidence of about 76%. In vitro, the product based on B. thuringiensis entailed a mycelial growth inhibition of less than 20%. This value is more than 75% higher than what was obtained through the other biological fungicides based on either T. harzianum or in B. subtilis. The efficacy of the latter 2 biological fungicides was more important in vivo assays using inoculated tomato plants. Indeed, by the use of the product based on B. subtilis, the reduction of disease incidence exceeded 95%. These results show that some biological fungicides can be used in controlling Fusarium crown and root rot of tomato.