Two structurally novel analogues of the macrolides archazolids A and B, archazolid A-15-O-β-D-glucopyranoside (archazolid E, 5) and iso-archazolid B (archazolid F, 6), were isolated from the myxobacterium Cystobacter violaceus and Archangium gephyra, respectively. Macrolactone 5 represents the first 15-O-glycoside of the archazolids. iso-Archazolid B (6) incorporates a C-3 alkene and presents the first constitutional isomer reported for this natural product class. The structures of these polyketides were determined by spectroscopic analysis, in particular by HMBC, HMQC, and ROESY NMR investigations and by chemical degradation. iso-Archazolid B (6) demonstrated extremely high antiproliferative and V-ATPase inhibitory effects, with IC(50) values in the picomolar range, while only moderate activity was observed for glycoside 5. iso-Archazolid B presents the most potent archazolid known.