The stramenopiles are a large and diverse group of eukaryotes that possess various lifestyles requiring adaptation to highly different niches. Among the stramenopiles, the MAST (MArine STramenopiles) clades are a diverse, polyphyletic assemblage of heterotrophic flagellates that are exclusively known from environmental sequences. Here we present the first cultivated representatives of MAST-3, which we name Mediocremonas mediterraneus gen. et sp. nov.