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Summary: Studies of the β-oxidation of the fatty acid side-chain of certain ω-aryl- and ω -aryloxy-n-alkylcarboxylic acids by Nocardia opaca (strain T16) and Nocardia sp. (strain P2) show that y-phenylbutyric is more rapidly oxidized than y-phenoxy- butyric acid. The effect of the oxygen bridge is even more striking when γ-(1-naphthyl)- and γ -(1-naphthyloxy)-butyric acids are compared. The rate of β -oxidation (by strain T18) of 3-, 4-, and 2-isomers decreases in that order. This applies to mono-chloro- and monomethylphenoxybutyric acids and to monochlorophenoxypropionic acids. Substitution in position 2 has by far the greatest effect and chlorine exerts a bigger influence than a methyl group in all positions. The conversion of γ-(2-methyl- 4-chlorophenoxy)-butyric acid (MCPB) and γ -(2:4-dichlorophenoxy)-butyric acid (2:4-DB) is very slow and proceeds only to β-hydroxy acids. On the other hand, ϵ-(2-methyl-4-chlorophenoxy)-, ϵ -(2:4-dichlorophenoxy)- and ϵ -(2-chlorophenoxy)-caproic acids are relatively rapidly converted to their corresponding butyric acid derivatives. With strain T16 ω-(2-naphthyloxy)-butyric and propionic acids are more rapidly converted to their corresponding acetic acids and phenols respectively than the ω-(1-naphthyloxy) compounds. The rate of β-oxidation of γ-phenyl-, γ-(3-indolyl)- and γ -(1-naphthyl)- butyric acids (by strain P2) decreases in that order. It has been shown that β-hydroxy acid intermediates are formed from ω-aryloxybutyric acids and those from MCPB, γ-(2-methyl-4-chlorophenoxy)-crotonic and γ-(2-naphthyl-oxy)-butyric acids have been isolated and identified.