Ergot alkaloids are a complex family of indol derivatives with the typical tetracyclic ergoline ringsystem, which are produced by fungi of two different families, e.g. Aspergillus fumigatus (A. fumigatus), from the family Trichocomaceae, and Claviceps purpurea (C. purpurea), from the family Clavicipitaceae. Ergot alkaloids exhibit different structures and therefore diverse biological activities. Based on the structures ergot alkaloids can be divided into three groups: clavine-type alkaloids, ergoamides and ergopeptines. Ergoamides and ergopeptines are amide and peptide derivatives of D-lysergic acid, respectively. Clavin-Type alkaloids consist merely of the ergoline ringsystem and are no D-lysergic acid derivatives. An example for a clavine-type alkaloid is fumigaclavine C, the end product of the ergot alkaloid biosynthetic pathway in A. fumigatus. Fungi of the family Trichocomaceae are producers of clavine-type alkaloids, whereas some members of the family Clavicipitaceae produce mainly ergoamides and ergopeptines. The comparison of the biosynthetic gene clusters of different ergot alkaloid producers led to identification of seven homologous genes. It was speculated that these genes are involved in the formation of the tetracyclic ergoline ring system. In previous studies, two of the seven homologous genes, fgaPT2 and fgaMT of A. fumigatus, were assigned to the corresponding step in the biosynthesis of the ergoline ring. The gene fgaPT2 encodes a prenyltransferase, which catalyzes the first step in the biosynthesis, i.e. prenylation of L-tryptophan. The product 4-dimethylallyltryptophan (4-DMAT) of this reaction is then methylated by N-methyltransferase FgaMT, resulting in the formation of N-methyl-4-dimethylallyltryptophan (4-DMA-L-abrine). In this thesis, functions of three additional genes, fgaDH, fgaOx3 and fgaFS of A. fumigatus, were proven biochemically. FgaDH contains the conserved motifs of classical short-chain dehydrogenases (SDRs), but shows otherwise no significant sequence similarity to SDRs or other known proteins. It was shown that FgaDH catalyses the oxidation of chanoclavine-I to chanoclavine-I aldehyde in the presence of NAD+. The structure of chanoclavine-I aldehyde was unequivocally elucidated by NMR and MS analysis. The KM value was determined as 0.27 mM for chanoclavine-I and 1.1 mM for NAD+, respectively The turnover number was determined as 0.38 s-1. The protein FgaOx3 contains an old yellow enzyme-like FMN binding domain, whereas FgaFS has no conserved regions or similarity to known proteins. FgaOx3 and FgaFS together are responsible for the formation of festuclavine from chanoclavine-I aldehyde. In the abscence of FgaFS a mixture of two stereoisomers is generated by FgaOx3 under consumption of oxygen. The isomers could not be further converted by FgaFS. The structures of these two stereoisomers as well as that of festuclavine were unequivocally elucidated by 1D- and 2D-NMR and MS analysis. On the basis of the obtained data, a reaction mechanism with chanoclavine-I aldehyde as substrate was postulated. Furthermore, a putative ergot alkaloid gene cluster was identified in fungi of the family Arthrodermataceae by analysis of genome sequences. The cluster contains five genes whose enzymes have homology to the enzymes, which are involved in the formation of chanoclavine-I aldehyde from L-tryptophan in A. fumigatus und C. purpurea. Homologues neither to the genes responsible for the conversion of chanoclavine-I aldehyde to festuclavine or agroclavine nor to the genes involved in the later steps of ergot alkaloid biosyntheses in A. fumigatus and C. purpurea exist in the genomes of these fungi. The fgaDH homologue from the putative ergot alkaloid cluster, ARB_04646, was amplified from gDNA of Arthroderma benhamiae (A. benhamiae), cloned and expressed in E. coli. The enzyme named ChaDH catalyses also the oxidation of chanoclavine-I in presence of NAD+. The enzymatic product was identified as chanoclavine-I aldehyde by NMR and MS analysis. The KM values were determined as 0.09 mM for chanoclavine-I and 0.36 mM for NAD+, respectively. The turnover number was found to be 0.76 s-1. Thus a third fungal family was identified in this thesis, which has at least the genetic information for the biosyntheses of ergot alkaloids or their precursors, i.e. chanoclavine-I aldehyde. But no ergot alkaloids were detectable in the supernatant of A. benhamiae cultures and the analysis of the expression of the five cluster genes by Northern blot showed no detectable expression under the used conditions.