Conglobatin C was isolated from a previously unreported Streptomyces strain, phylogenetically distinct from the reported conglobatin A producer, Streptomyces conglobatus. Conglobatins B and C diverge from conglobatin through differing patterns of methylation on the macrodiolide skeleton, suggesting a deviation from the published biosynthetic pathway, which proposed three successive methylmalonyl-​CoA extender unit additions to the conglobatin monomer.  Conglobatins B and C exhibited more potent cytotoxic activity selectively against the NS-​1 myeloma cell line compared with conglobatin.