Novel insecticidally active heterocyclic compounds of the formula ##STR1## wherein W represents a substituted pyridyl group, or a 5- or 6-membered optionally substituted heterocyclic group containing at least two hetero atoms selected from oxygen, sulfur and nitrogen atoms,R represents a hydrogen atom or an alkyl group,Y represents .dbd.N--or ##STR2## R' represents a hydrogen atom, an alkyl group, an aryl group, an acyl group, an alkoxycarbonyl group or a cyano group,Z represents a nitro group or a cyano group, andT represents 3 or 4 optionally substituted ring members of a 5- or 6-membered unsaturated heterocyclic ring which it forms together with the adjoining carbon atom and nitrogen atom, said 5- or 6-membered unsaturated heterocyclic ring containing 1 to 3 hetero atoms which are selected from oxygen, sulfur and nitrogen atoms and at least one of which is a nitrogen atom.