In our continuing studies on toxigenic food-borne fungi, isolates of Aspergillus caespitosus Raper and Thorn were investigated. The isolate used was obtained from the Centraalbureaux voor Schimmelcultures, Baarn (C.B.S. 246.73) and deposited in the culture collection of the S.A. Medical Research Council as MRC 271. When cultivated on whole yellow maize, this isolate caused a neurotoxicity syndrome in rats characterized by sustained tremors and other neurological signs. This paper relates the isolation of three alkaloids from A. caespitosus (MRC 271), viz. fumitremorgen B(l), TR2 (3) and cyclopiamine B (4), and constitutes the first report on the natural occurrence of TR2 and the production of cyclopiamine B by an Aspergillus species. Cyclopiamine B is known to be produced by Penicillium cyclopium Westling and Penicillium urticae Bainier only, while TR2 was previously derived from the hydrogenation of the secondary-tertiary dialkyl peroxide, verruculogen in the presence of palladium on carbon. The foregoing alkaloids were fully characterized by direct comparison (spectroscopic and chromatographic properties) with reference samples. The co-occurrence of fumitremorgen B (1) and TR2 (3) in cultures of A. caespitosus established their biosynthetic relationship. Fumitremorgen B, TR2 and cyclopiamine B seem to be biogenetically derived from tryptophan, proline and two units of dimethylallyl-pyrophosphate, with main structural differences originating from the different linkages of the two C5-units which lead to the two novel structures.