Abstract: In order to study pathogenicity of the fourth RTX toxin ApxIV in the infection of Actinobacillus pleuropneumoniae, an Actinobacillus pleuropneumoniae apxIVA mutant was constructed by transconjugation and counterselection method,using an A. pleuropneumoniae HB04 strain (serotype 7) as parental strain. The apxIV mutant was confirmed by PCR, sequence analysis, Southern blot and heredity stability assays. The biological characteristics of this mutant strain was further investigated. The mutant possesses the same growth rate as its parental strain, but it was less virulent on mice. These results indicated that apxIV was an important virulence factor in the pathogenesis of A. pleuropneumoniae infection. This study have a further illumination on the pathogenicity of A. pleuropneumoniae and it will facilitate the development of more reasonable live attenuated vaccines of APP.