New 16-membered macrolide antibiotics 6108A, 6108B, 6108C, and 6108D I [R = CH:NNHAc, CO2H, CH(OH)SCH2CH(NH2)CO2H, CH:CHCOMe], resp., were isolated from a culture filtrate of M. fastidiosus and their physico-chem., chem. degradation studies, and spectroscopic data confirmed their identity as rosmaricin (I; R = CHO) analogs.Wittig reaction of the latter with Ph3P:CHCOMe gave a product identical with I (R = CH:CHCOMe).