Chitinase is an essential enzyme for the insect ecdysis. Inhibitors of chitinase would be expected to interrupt insect moulting and thus prevent maturation to adult reproductive stage. In the course of screening for chitinase inhibitors, we have found new compounds named argifin (1) and argadin (2) from the cultured broths of Gliocladium sp. FTD-0668 and Clonostachys sp. FO-7314, respectively. The structures of 1 and 2 were elucidated by amino acid analysis, NMR experiments including ^1H-^<13>C and ^1H-^<15>N HMBC, and other spectroscopic analyses. The structure of 1 was shown to be cyclo(N^ω-(N-methylcarbamoyl)-L-arginyl-N-methyl-L-phenylalany1-β-L-aspartyl-β-L-aspartyl-D-alanyl). The β-amide bonds of two aspartic residues were revealed by highfield shifts of free carboxylic carbons in acidic solution. The structure of 2 was deduced to be cyclo(N^ω-acetyl-L-arginyl-D-prolyl-homoseryl-histidyl-L-2-aminoadipyl) in which homoseryl γ-methylene bonded to histidyl α-amino residue to form a γ-lactam. The γ-lactam structure was confirmed by small shifts of homoseryl carbonyl carbon measured in D_2O alone and H_2O-D_2O, which suggested it bonded tertiary nitrogen. The IC_<50> values of 1 and 2 against Lucilia cuprina (blowfly) chitinase at 20℃ were 100 and 3.4nM, respectively. Their inhibitions were reduced about forty times at 37℃. They are the first chitinase inhibitors produced by fungi and showed inhibition in a submicromolar range. They arrested the moult of cockroach larvae upon injection into the ventral abdominal part. Therefore, cyclic pentapeptide containing N^ω-substituted-L-arginine could be an interesting lead for the development of novel insecticides.