Disclosed are compounds of Formula 1, being mesoionic pyridinium compounds containing a five-membered ring fused to the pyridinium ring. Specific examples of compounds of Formula 1 include: 9-bromo-1-[(2-chloro-5-thiazolyl)methyl]-2-hydroxy-4-oxo-3-[3-(trifluoromethoxy)phenyl]-4H-pyrido[1,2-a]pyrimidinium inner salt, 2-hydroxy-9-methyl-1-[(2-methyl-5-thiazolyl)methyl]-4-oxo-3-[3-(trifluoromethyl)phenyl]-4H-pyrido[1,2-a]pyrimidinium inner salt, and 1-[(2-chloro-5-thiazolyl)methyl]-3-(2,5-difluoro[1,1-biphenyl]-3-yl)-2-hydroxy-9-methyl-4-oxo-4H-pyrido[1,2-a]pyrimidinium inner salt. Compounds of formula 1 are useful for controlling invertebrate pests such as arthropods and for treating parasite infections in animals or infestations in the general environment.
