A soil enrichment technique was used to isolate microorganisms which could degrade ioxynil (3,5-diiodo-4-hydroxybenzonitrile). Many isolates obtained were able to degrade ioxynil to various products. However, only a fungal isolate (Fusarium solani) and a Gram-negative bacterium (Klebsiella ozaenae) released 14CO2 from ring-labeled ioxynil. No appreciable degradation was detected in pure cultures without the addition of exogenous nutrients. Results indicated that the degradation of ioxynil to CO2 proceeded more slowly in pure culture. Ioxynil was degraded in pure culture at a faster rate by F. solani than by K. ozaenae. Analyses of radioactivity distribution in the cultures indicated that a sizable fraction of radioactivity was in the form of polar products. Several degradation products were detected in the ethyl acetate extracts by thin-layer chromatography and subsequent radioautography. Screening of pure cultures of ioxynil degraders revealed that most isolates degraded ioxynil to the same products which were extractable with ethyl acetate.