The structures of two solid-state rearrangement products, A and B, of rosaramicin have been assigned on the basis of 13C NMR spectral data and a single-crystal X-ray analysis of the acetone solvate of product B.Rosaramicin 1, a sixteen-membered ring macrolide antibiotic produced by Micromonospora rosaria, is highly active against Gram-positive as well as Gram-negative bacteria. *~5) Whenthe formulated tablets* of 1 were subjected to heating at 75°C for extended periods of time (2-4 weeks), high pressure chromatographic analysis of the resulting material indicated that approximately 20% degradation had occurred. Tablets treated in this manner were ground and extracted with methanol and chloroform. Concentration of the combined extracts followed by chromatography of the resulting residue on silica gel furnished two major products, A (33%) and B (9%), the structures of which are reported here. The major degradation product A, obtained in excellent purity, had the same MW(m/z 581) as 1. Its XHNMRspectrum [(CD3)2CO] showed the presence ofmethyl proton resonances at S 0.88 (t, /= 7.0 Hz),