Thirty-six microorganisms were screened for their abilities to transform (+)-catechin to metabolites. Of these, Aspergillus giganteus UI 10, A. ochraceous ATCC 1008, Cylindrocarpon radicicola ATCC 11011, Amycolata autotrophica ATCC 35203, Mycobacterium flavescens ATCC 14474, M. fortuitum UI 53378, Streptomyces rimosus NRRL 2234, S. griseolus ATCC 11796, and S. griseus ATCC 13273, converted (+)-catechin to new products. C. radicicola and S. griseolus were chosen for preparative-scale incubations to produce polar products, which were isolated and characterized by UV, NMR, and mass spectral analyses. The products were new carboxylic acid lactones formed by B-ring fission of catechin. Labeling with 18O2 showed incorporation of two oxygen atoms into the new lactone products. Based on 18O2 labeling, likely pathways for lactone formation involved initial dioxygenase-mediated meta B-ring cleavage followed either by aldehyde oxidation to a diacid that lactonizes, or by hemiacetal (lactal) formation followed by alcohol oxidation. We believe this to be the first example of microbial B-aryl-ring cleavage in catechins.