Chemical structures of new piericidins produced by Streptomyces pactum are elucidated on the basis of mass, PMR and CMR spectral analyses. Consequently, these piericidins were shown to be constructed by a combination of variations in each four functionalities and carbon skeletons. During screening of metabolites of microorganisms for new insecticides, we found fourteen new piericidins and two known ones, PA1 and PB1, in the mycelial extract of Streptomyces pactum. In the preceding paper, we described the isolation procedure of these piericidins which were classified into four groups, PA, PB, PC and PD, according to their chromatographic behavior. Here we wish to report the elucidation of the chemical structures of the new piericidins by consideration of their structural relations to the known PA1 (I) using mass, PMR and CMR spectro metries. The chemical structure of PA1 has been established mainly on the basis of extensive degradation studies. On the other hand, recent developments in various spectro metries enable us to determine the structure of these compounds by a few chemical degradation studies. First, we attempted the structural determination of the PC series, since these compounds were obtained in a fairly large amount and were thought to have a novel functional type. The molecular formula of PC1, C25H37NO5, determined from the high resolution mass spectrum, indicates that PC1 contains one more oxygen atom than PA1. The mass spectrum of PC1 shows strong fragment peaks at m/e 331 and 330, and the pattern in the low mass region below these peaks is similar to that of PA1. This suggests both PA1 and PC1 have the same partial structure in the structural part of the substituted pyridine ring and the side chain from C-1 to C-9, because the strong peaks are generated by cleavage of the bond between C-9 and C-10. In the CMR spectrum, 13C chemical shifts of PC1 resemble those of PA1 except for C-11 and C-12. Therefore, the structure of PC1 must be different from PA1 only in the terminal part of the side chain, which is composed of C5H9O2. The offresonance CMR spectrum reveals that the terminal moiety of PC1 consists of two methyl carbons (-CH3), two carbons having a hydrogen and an oxygen and one carbon attached to an oxygen. The CMR data together with the observation that one unsaturation is involved.