A new fungal isolate that grows endophytically in sweet sorghum was identified as Helminthosporium velutinum Link ex Ficinus & Schubert. Light-microscopy of cross-sections of colonized sweet sorghum roots showed that the intercellular, pigmented hyphae of the fungus was mostly limited to the epidermal layer and formed outer mantle-like structures. This endophyte has the ability to significantly increase sweet sorghum biomass. This is the first report of Helminthosporium as an endophyte and could help realize sustainable the biomass production for biofuel purposes.Key words: endophyte, Helminthosporium, sweet sorghumThere is growing interest in biofuel production as an alternative to non-renewable energy sources (2). All renewable fuels of biological origin including fuel wood, bioalcohol and biogas, and purpose-grown crops such as sugarcane, maize, sweet sorghum, and rape seeds, represent an important source of biomass to satisfy increasing demand, raising hopes but also concerns related to competition (water and land) for food production. Assigning abandoned or nonarable lands to the production of purpose-grown biomass could help eliminate risks. Such a strategy would require growth-promoting measures.Dark septate endophtytic fungi (DSE) live in symbiosis with certain plants, improving the ability of these plants to tolerate unfavorable conditions (8,14). When isolated in pure cultures, colonies of DSE are generally non-descript, range in color from olivaceous to brown or almost black, and often lack conidia or other taxonomically distinctive characteristics (9). Taxa that have sporulated in culture include Phialocephala fortinii Wang & Wilcox, Phialocephala sphaeroides Wilson, Cadophora finlandica (Wang & Wilcox) Harr & McNew (as "finlandia"), Leptodontidium orchidicola Sigler & Currah and Heteroconium chaetospira (Grove) Ellis. P. fortinii is the most commonly reported representative of this group. All these fungi have the ability, through mechanisms such as the transfer of nitrogen and uptake of phosphorus, to provide nutrients to host plants (6,15,16). The group has a broad host range encompassing approximately 600 plant species, suggesting little or no host specificity (9). H. chaetospira is able to colonize the roots of plants from eight families (4, 12). Members of the Gramineae family are no exception, but one gramineous crop, barley, is not suitable for H. chaetospira and P. fortinii which have little the ability to grow in the roots of axenically grown plants (12).The purpose of the present study was to identify fungal endophytes which promote the growth of their host plants, particularly sweet sorghum. To obtain such endophytes from the natural environment, it is necessary to choose an appropriate method of isolation with respect to the targeted parts of plants. Therefore, surface sterilization was used to isolate fungal endophytes from living roots, leaves-stems and winter buds, known to be inhabited endophytes (1, 7, 10). We performed a pathogenicity test as a first screening for endophytes and exa...