Parasitella parasitica, a facultative mycoparasite of zygomycetous fungi, forms cytoplasmic fusions with its hosts during infection. Thus, the organism is an efficient donor of genetic material in parasexual host-parasite interactions. Recognition between parasite and host is mediated by trisporoids, which are also responsible for sexual communication. The TDH gene for one of the key enzymes of trisporic acid biosynthesis, 4-dihydromethyl-trisporate dehydrogenase, was cloned and its transcription analysed. TDH was cloned on a 6175-bp insert and was found to map in a complex cluster of genes that suggest post-transcriptional antisense regulation. Histochemical TDH analysis in developing parasitic or sexual structures shows high enzymatic activity in Parasitella. TDH is linked to a gene for a putative acyl-CoA thioesterase (ACT). Two ORFs were identified in the 5'-region of the TDH gene, a third one, coding for 176 amino acids overlaps the ACT gene in antisense direction completely. Expression levels of ACT and ORF1 depend on parasitic and sexual interactions.