The invention relates to a cell which comprises a nucleotide sequence encoding a xylose isomerase, wherein the amino acid sequence of the xylose isomerase has at least about 70% sequence identity to the amino acid sequence set out in SEQ ID NO: 3 and wherein the nucleotide sequence is heterologous to the host. The cell of the invention may be used in a process for producing a fermentation product, such as ethanol. Such a process may comprise fermenting a medium containing a source of xylose with a cell of the invention such that the cell ferments xylose to the fermentation product.