Mesoionic imidazolium compounds of formula (I) and their uses for combating animal pests. The present invention relates to compounds of formula (I), wherein A, W, Y, T and R1 are defined as in the description, and to the stereoisomers, salts, tautomers and N-oxides thereof and to compositions comprising such compounds. The invention also relates to methods and uses of these compounds and of compositions thereof, for combating and controlling animal pests. Furthermore the invention relates also to pesticidal methods of applying such substituted pyrimidinium compounds.