1. Anthelmintic effect in vitro against human and hog ascaris was examined with a total of 27 kinds of 4-alkyl-, 4-alkyl-6-chloro-, 4-alkyl-6-bromo-, and 4-alkyl-2, 6-dichloro-resorcinols in 1:1,000 to 1 2,000 dilutions. Based on the results thereby obtained, 14 kinds of compounds of alkylresorcinol and alkylmonochlororesorcinol series were submitted to anthelmintic treatment in man and their effect was examined by the egg-counting and worm-counting methods. 2. The in vitro effect of alkylresorcinols and their monochloro derivatives was found to be the strongest in hexylresorcinol and became weaker with a larger or smaller number of carbon atoms in the alkyl group. Comparison of the corresponding compounds showed that the alkylresorcinols generally possessed a stronger action than their monochloro derivatives. 3. Anthelmintic effect of alkylrescrcinols in man was also the strongest in the hexyl compound, followed by amyl- and heptyl-resorcinols. In the monochloro derivatives, their anthelmintic effect did not follow the order of their in vitro effect, effect, stronger than hexylresorcinol (m. p. 68-69°) being shown by amyl-(m. p. 43-44°), hexyl-(m. p. 46-48°), heptyl-(m. p. 45-47°), and octyl-chlororesorcinols (m. p. 49-51.5°), as well as by cyclo-hexylchororesorcinol (m. p. 86-88°), whose non-chlorinated compound was totally ineffective. 4. Ascaridical effect of alkylresorcinols decreased by the introduction of one bromine or two chlorine atoms.