The invention provides a method of combating fungus at a locus which comprises treating the locus with a compound of the general formula

or an N-alkyl or N-phenyl halide or N-oxide thereof, in which R¹, R², R³ and R⁴ independently represent a hydrogen or halogen atom, a hydroxyl group or an optionally substituted alkyl or alkoxy group, or R¹ and R² or R² and R³ or R³ and R⁴ together with the interjacent carbon atoms represent a 5- to 7-membered saturated or unsaturated carbocyclic or heterocyclic ring in which a heterocyclic ring contains 1 to 3 heteroatoms selected from nitrogen, oxygen and sulphur atoms; R⁵ represents a hydrogen or halogen atom, R⁶ represents a hydrogen atom, or R⁵ and R⁶ together represent a single carbon- carbon bond; R⁷ represents a hydrogen or halogen atom, an alkyl or alkoxy group optionally substituted by one or more halogen atoms, or an optionally substituted phenyl or phenoxy group; R⁸ and R⁹ independently represent a hydrogen atom or together represent a single carbon-carbon bond; and A represents an optionally substituted phenyl group. Certain of the isoquinoline derivatives are novel and a process for the preparation of these compounds and compositions containing them are also provided.