Filamentous fungi have the genetic capacity to produce a huge array of secondary metabolites (SMs) contributing to their fitness and survival. Some of them are beneficial for human kind having a pharmaceutical relevance whereas others are detrimental for livestock and mankind upon consumption. The majority of genes required for the metabolite production are generally organized in gene clusters located in subtelomeric regions, thus being prone to chromatin modifications. Furthermore, theses gene clusters are often silent under laboratory conditions, thereby hampering the discovery of novel SMs. Recently it has been shown that posttranslational histone modifications involved in chromatin remodelling modulate the expression pattern of SMs. One such modification is methylation regulating a wide range of cellular processes whereas the counterpart of methylation is carried out by demethylases. This work characterizes a recently discovered JmjC domain containing demethylase, KdmB, encoded by the gene AN8211, which targets the Histone 3 Lysin 4 trimethyl mark (H3K4me3) in Aspergillus nidulans. Originally KdmB was thought as a repressor upon demethylation of the activating H3K4me3 mark. However, upon deletion of KdmB a high number of SM genes are downregulated, thus suggesting that KdmB has a role in gene activation as well .By mutation of the highly conserved catalytic JmjC domain the demethylase activity was dissected from the rest of the protein, giving a further insight into the catalytic mechanism of KdmB. As a control, the kmdB deletion strain was complemented by in loco integration of kdmB driven by its native promoter (KdmBCil). Next, the expression of sterigmatocystin and penicillin was analysed. Surprisingly, while expression and production levels were rescued to wild type level in KdmBCil, this was not the case for the mutated strain. Thus, we could show that the demethylation by KdmB is required for the activation/repression of SM genes in A. nidulans.