Abstract [U- 14C]Pentachloronitrobenzene (purity >99.5%) was applied to soil (36.1 kg/ha) under outdoor conditions, and onion bulbs were planted. At harvest, the peeled onion bulbs contained <0.1 μg/g radioactive substances (equivalent to pentachloronitrobenzene), roots and peels 98.4 μg/g, and shoots 31.7 μg/g. About one-half to two-thirds of these residues were extractable by cold chloroform/methanol (1:1). The total metabolism rate was about 60% in the shoot extract and 80% in the peel-root extract. With the aid of authentic reference compounds, the following metabolites were identified from the shoot extract: pentachloroaniline (I), pentachlorophenyl-methylsulfoxide (II), pentachlorothioanisole (III), pentachlorobenzene (IV), and a tetrachloroaniline (V). From the root-peel extract, the following conversion products were identified: pentachloroaniline (I), pentachloroanisole (VI), pentachlorophenylmethylsulfoxide (II), pentachlorothioanisole (III), pentachlorobenzene (IV), pentachlorothiophenol (VII), pentachlorophenol (VIII), pentachlorophenylmethylsulfone (IX), pentachlorophenylacetate (X), two isomers of tetrachlorobenzene (XI and XII), and two isomers of tetrachloronitrobenzene (XIII and XIV). Additionally, structures are proposed for four minor conversion products (XV–XVIII) based on their mass spectra. These metabolites are derived from metabolites I–XIV by dechlorination, formylation, or introduction of the thiomethyl group. The residues extracted from peeled onion bulbs were too low for characterization.