The present invention relates to methods of producing a C4 dicarboxylic acid, comprising: (a) cultivating a filamentous fungal host cell comprising a polynucleotide selected from the group consisting of a heterologous first polynucleotide encoding a C4 dicarboxylic acid transporter, a heterologous second polynucleotide encoding a malate dehydrogenase, and a heterologous third polynucleotide encoding a pyruvate carboxylase; wherein the filamentous fungal host cell is capable of secreting increased levels of the C4 dicarboxylic acid compared to the filamentous fungal host cell without the heterologous polynucleotide when cultivated under the same conditions; and (b) recovering the C4 dicarboxylic acid. The present invention also relates to methods for increasing C4 dicarboxylic acid production, filamentous fungal host cells and malate dehydrogenase variants.