In our continuing search for insecticidal substances from microbial metabolites, we recently found two active compounds from Penicillium: okaramine A (1), a new type of indole alkaloid with a seven-ring system, and okaramine B (2), a derivative with an eight-ring system, both showing marked insecticidal activity toward the 3rd instar larvae of silkworm upon oral administration. To determine if okaramines or similar compounds were produced by other strains, we studied Penicillium simplicissimum AHU8402, which showed activity almost equivalent to AK-40 (the producing strain of okaramines). This paper reports the isolation and structural elucidation of the insecticidal constituents of AHU8402. From an acetone extract of mycelia and media of this strain, three insecticidal compounds were isolated. Two of them were identified as okaramine A (1) and okaramine B (2), and the third was a new congener termed okaramine C (3). High-resolution EI mass spectrometry established 3's molecular formula as C32H36N4O3. 1H-NMR and 13C-NMR spectra revealed 3 contains 3a-hydroxy-N8-(α,α-dimethylallyl)-1,2,3,3a,8,8a-hexahydropyrrolo[2,3-b]indole-2-carboxylic acid and a 2,3-disubstituted indole with a diketopiperazine ring, differing from 1 only in the azocine ring. 3 is a tetrahydro derivative of 1, with a saturated C1'-C2' double bond and reductively cleaved N3'-C4' bond (supported by COSY, UV, and 1H-NMR data). The relative configuration of the pyrrolidine ring in 3 is the same as in 1; the configuration at C2' remains uncertain. Okaramines exhibit insecticidal activity against silkworm larvae upon oral administration: LD50 values are 8 µg/g diet (1), 0.2 µg/g diet (2), and 7 µg/g diet (3), suggesting the azocine ring moiety does not play an essential role in activity.