Abstract: Dark septate endophytes (DSEs) are ascomycetous root colonizer fungi distributed widely, and commonly found in environments with strong abiotic stress. DSEs are regarded as potential plant promotion agents and were shown to help plants against fungal diseases. Besides being ubiquitous, the ecological function of DSEs is not well understood. Although they co-occur in the root with beneficial plant microbes, our knowledge e.g. about the interaction between DSEs and other common fungi, such as the arbuscular mycorrhizal fungi (AMF) is poorly understood. Considering that antagonism with AMF is a negative prosperity of inocula developed for agricultural use, it is important to test whether fungi, that show potential as bio-fertilizers, would have negative effect on plant colonization by other beneficial microbes. In the current study, we performed a greenhouse experiment using tomato plants to study the interaction between two model DSEs Periconia macrospinosa and Cadophora sp., and the AM fungus Rhizoglomus irregulare. The effect of inoculation of AMF on plants was evaluated with and without the DSEs, and the colonization rate of AMF between the treatments was assessed using microscopy and qRT-PCR assays. The results show that the DSEs did not influence the effects of the AMF Rhizoglomus irregulare on the host plants.