Beauvericin 111 and two novel cyclodepsipeptide analogues, beauvericin A 121 and beauvericin B 131, containing 2-hydroxy-3-methylpentanoic acid as one of the constituent residues, were isolated as insect toxic principles from a mycelial extract of the fungus Beauveria bassiana. Cyclodepsipeptide toxin beauvericin 111 is known to be produced by several fungi, including Beauveria bassiana (Hyphomycetes) (1-8). During a screening program aimed at identifying extracts with biological activity of agrochemical interest, an isolate ofB. bassiana showed toxic activity against mosquito larvae. Bioassay-guided separation resulted in the isolation of 1 as the most abundant bioactive component. However, all the activity could not be accounted for by 1 alone and further analysis resulted in the isolation of two novel analogues of 1. One of these compounds was fully characterized, mostly by the analysis of its nmr data, as the cyclodepsipeptide 2 (beauvericin A).