Members of the class Actinobacteria are known to produce pharmaceutically important compounds, but the rate of discovery of novel compounds from soil Actinobacteria strains has decreased significantly. Therefore, we explored the possibility of isolating unusual Actinobacteria capable of producing new metabolites by employing the membrane filter (MF) method and isolated Streptomyces sp. RI18 from soil. From its culture, we isolated two novel benzastatin derivatives, JBIR-73 (1) and JBIR-67 (2), along with the known benzastatin derivatives virantmycin and 7-hydroxyl benzastatin D. The structure of 1 was elucidated by 2D NMR analyses to be a derivative of 7-hydroxyl benzastatin D with the hydroxyl residue at C-9 replaced by an ergothioneine moiety (representing the first reported benzastatin derivative containing an S-hercynine moiety), and 2 was determined to be a 1-carboxyl benzastatin G (carboxylic acid form of benzastatin G). Activity evaluations showed these compounds were weakly cytotoxic to human cervical carcinoma HeLa cell lines (IC50 > 50 μM) but did not exhibit antimicrobial activity against Candida albicans, Micrococcus luteus, or Escherichia coli. This study suggests that Actinobacteria isolated using the MF method may serve as a source of new compounds with unique skeletal structures.