Concanamycins, isolated from the mycelium of Streptornyces diastatochromogenes S-45, inhibit the proliferation of the mouse splenic lymphocytes stimulated by concanavalin A. Concanamycin A (1a) was previously revealed to be the second example of the 18-membered macrolide antibiotics following borrelidin. In this communication, the structures of concanamycins B (1b) and C (1c) are described. Concanamycin B (1b), an amorphous white powder with molecular formula C45H73NO14, was analyzed via treatment with 0.03 N NaOH in methanol (affording anhydroaglycone P2 and sugar S1), extensive PMR analysis, CMR spectrum, and ozonolysis followed by reduction with dimethyl sulfide, revealing that an ethyl group at C(8) of concanamycin A's anhydroaglycone (2a) was replaced by a methyl group and a 6-membered hemiketal ring was formed by the 21-carbon and the 25-hydroxyl. Concanamycin C (1c), which crystallized from ethanol as colorless thin plates with molecular formula C45H74O13, was analyzed via alkaline degradation (affording anhydroaglycone P1 and sugar S2 identified as 2-deoxy-D-rhamnose) and NMR spectra, suggesting it is a decarbamyl derivative of concanamycin A with a hemiketal ring structure. Thus, concanamycins A, B and C are novel 18-membered macrolide antibiotics consisting of an α,β,γ,δ-unsaturated lactone ring, a long side chain which forms an intramolecular hemiketal ring, and 2-deoxy-D-rhamnose.