The bacterial thyA gene encodes the enzyme thymidylate synthase, which is essential for dTMP synthesis and, consequently, for DNA replication. In this work, a Haemophilus parasuis thyA mutant was constructed in order to analyze its colonization characteristics and its capacity to generate serum bactericidal activity in infected guinea pigs. The data showed that colonization by the H. parasuis thyA mutant was much less than that of the wild-type strain. Nevertheless, the mutant generated a strong immunogenic response in the host, as detected by measuring serum bactericidal activity.