In the course of screening for antifolic metabolites using Lactobacillus casei as a test organism growing on a synthetic medium with a limiting amount of folic acid, two peptide antibiotic cirratiomycin A (1) and B (2) have been isolated from the culture filtrate of Streptomyces cirratus No. 248. In this paper, we describe the isolation and structural elucidation of these antibiotics.