While screening fungal cultures of diverse origin for new bioactive metabolites against Paenibacillus larvae (the causative agent of American foulbrood in honeybees), we investigated a strain of the terrestrial fungus Gliocladium catenulatum. From its mycelial extract, we isolated a previously undescribed biindol metabolite 1, along with two known ETP-derived compounds 2 and 3. Structural analysis using techniques such as 2D NMR and HR-MS elucidated the structure of compound 1 as 3,3¢-biindole. Compound 1 showed specific antimicrobial activity against P. larvae, with an inhibition halo diameter of 15 mm (50 mg per disk) and a MIC of 5 mg per disk. Compounds 2 and 3 were identified as the known gliocladins A and C based on their spectroscopic data. Compound 1 had not been reported as a natural product before (only as a synthetic compound), and this is the first report of isolating these compounds from G. catenulatum.