Extraction of mycelium from cultures of Penicillium roqueforti (strain CS1) yielded 2 new crystalline metabolites, designated roquefortine and isofumigaclavine A. The structure of roquefortine, the major alkaloid, was established by a combination of chemical transformations and spectroscopic studies. The structure of isofumigaclavine A was determined by spectroscopic and X-ray analysis. Roquefortine possessed neurotoxic properties when injected intraperitoneally in mice and has been detected in 16 out of 16 samples of blue cheese from 7 countries. Isofumigaclavine A has also been found in blue cheese.