NOVELTY - Heterocyclic compounds (AA) and their salts are new.    USE - (AA) are useful for controlling unwanted nematodes in mammals, birds or their food, plants, seeds or soil (claimed), where: the mammal is domesticated animal, e.g. a companion animal (e.g. a cat, dog, horse or rabbit) or livestock (e.g. a cow, sheep, pig, or goat) or a human; the plants are cotton, strawberries, tomatoes, potatoes, vegetables or ornamental plants; and the nematodes are e.g. Ditylenchus, Tylenchorhynchus, Pratylenchus, Hirschmanniella or Aphelenchoides.    ADVANTAGE - (AA) have high potency, a broad spectrum of activity against different strains of nematodes and are environmentally safe or non-toxic to non-target organisms.    DETAILED DESCRIPTION - Heterocyclic compounds (AA) comprising heteroaryl compounds of formulae (I)-(VI), (XIX) and (XX), oxadiazole compounds of formulae (VII)-(XII) and thiadiazole compounds of formulae (XIII)-(XVIII) and their salts, are new.    A1 = aryl, arylalkyl, aryloxo, arylthio, heteroaryl (including pyridyl, pyrazyl, oxazolyl or isoxazolyl), heteroarylalkyl, heteroaryloxo or heteroarylthio (all optionally substituted with halo, 1-6C-haloalkyl, 6-10C-aryl, 4-7C-cycloalkyl, 2-6C-alkyl, 2-6C-alkenyl, 2-6C-alkynyl, 6-10C-aryl(1-6C)alkyl, 6-10C-aryl(2-6C)alkenyl, 6-10C-aryl(2-6C)alkynyl, 1-6C-hydroxyalkyl, amino, ureido, cyano, 1-6C-acylamino, hydroxy, thiol, 1-6C-acyloxy, azido, 1-6C-alkoxy or carboxy) or C(H)O;    B1 = C(H) or C(CH3);    C1 = heteroaryl including thienyl, furanyl, oxazolyl or isoxazolyl (all optionally substituted with at least one of F, Cl, CH3 or OCF3), or pyrrolyl (including pyrrolyl-1, pyrrolyl-2 or pyrrolyl-3), pyrrolyloxo (including pyrrolyl-2 or pyrrolyl-3), pyrrolylthio (including pyrrolyl-2 or pyrrolyl-3) or pyrrolylalkyl (including pyrrolyl-1, pyrrolyl-2 or pyrrolyl-3) (all optionally substituted by methyl, halomethyl, alkyl, haloalkyl, cyclyl, halo, heterocyclyl or hydroxyalkyl); and    X = O or S.    INDEPENDENT CLAIMS are included for:    (1) a method for controlling unwanted nematodes comprising administering a composition comprising (AA) to mammals, birds or their food, plants, seeds or soil; and    (2) a nematicidal composition comprising (AA).