A composition for killing harmful organisms, which contains a clathrate compound composed of one or more natural essential oils and/or derivatives thereof selected from the group consisting of the following substances (1) to (22) and a polymolecular host compound capable of including the natural essential oils and/or derivatives thereof:(1) hinoki oil;(2) 1,8-cineole;(3) l-.alpha.-terpineole;l-carbone;(5) l-menthone;(6) d-pulegone;(7) citronellal;(8) linelool oxide;(9) d,l-citronellol;(10) 3,3,5-trimethylcyclohexanol;(11) l-perillaldehyde;(12) l-carveol;(13) myrtenal;(14) peppermint oil;(15) eucalyptus oil;(16) cinnamic aldehyde;(17) .alpha.-bromo-cinnamic aldehyde;(18) salicylaldehyde;(19) benzaldehyde;(20) paraphenyl-propionaldehyde;(21) paratolualdehyde; and(22) l-menthol.