Six novel bioactive bicyclic polyketides (1-6) were isolated from cultures of an endophytic fungus of the medicinal plant Globularia alypum collected in Batna, Algeria. The producer organism was identified as Preussia similis using morphological and molecular phylogenetic methods. The structures of metabolites 1-6, for which the trivial names preussilides A-F are proposed, were elucidated using a combination of spectral methods, including extensive 2D NMR spectroscopy, high-resolution mass spectrometry, and CD spectroscopy. Preussilides were tested for antimicrobial and antiproliferative effects, and, in particular, compounds 1 and 3 showed selective activities against eukaryotes. Subsequent studies on the influence of 1 and 3 on the morphology of human osteosarcoma cells (U2OS) suggest that these two polyketides might target an enzyme involved in coordination of the cell division cycle. Hence, they might, for instance, affect timing or spindle assembly mechanisms, leading to defects in chromosome segregation and/or spindle geometry.