The plant growth-regulating activities of homologous series of $\omega $-(4-chlorophenoxy)-, $\omega $-(2:4-dichlorophenoxy)- and $\omega $-(2:4:5-trichlorophenoxy)-alkylcarboxylic acids and of the corresponding $\omega $-(1-naphthyl)- acids have been assessed in the wheat cylinder, pea curvature and tomato-leaf epinasty tests. In most instances, an alternation in activity was observed as each series was ascended. With some series, however, although the acetic derivative was active, all higher homologues were inactive in certain tests. The results are discussed and shown to be consistent with a possible degradation of the side-chain of these acids by $\beta $-oxidation within the plant tissue. Although the evidence indicates that $\beta $-oxidase enzyme systems are present in all three types of plant tissue employed, whether or not such enzymes can degrade the side-chain of these acids appears to be related to the type of ring system and to the orientation of nuclear substituents present in the molecule. Thus, for example, 4-chlorophenoxy- and 2:4-dichlorophenoxy-acetic acids and their alternate homologues were active in all three tests, whereas in the case of the $\omega $-(2:4:5-trichlorophenoxy)- series, this typical alternation in growth-regulating activity was shown only in the wheat cylinder test. However, when solutions of alternate homologues of the latter series were pre-treated with wheat cole-optile tissue, they induced growth responses in the pea test. Furthermore, the pre-treated solution of the butyric derivative was shown by chromatographic methods to contain the acetic homologue. The growth-regulating activity of a number of 1-naphthyl-acids with alkyl substituents in the side-chain was also determined. The results are consistent with the view that substitution in the $\beta $-position of the side-chain can hinder $\beta $-oxidation of these compounds.