The development of complement in fish is poorly understood, and little information is available about the acquisition of full complement capacities. In this study we demonstrated the ontogenic and differential expression of complement key component genes (C3, C1r/s, C4, C6, Bf, MBL and MASP) of the classical pathway (CP), alternative pathway (AP), lectin pathway (LP) and lytic pathway, and their responses to challenge with LPS during development. All components, except C6, showed a similar trend of increasing expression after hatching. The contents of Bf and C3 transcripts were both markedly increased by challenge with LPS soon after hatching, while the expression levels of C1r/s and MASP transcripts were not elevated. The expression of C4 mRNA was first down-regulated by LPS challenge and then fluctuated. The expression of MBL mRNA was up-regulated by LPS challenge after 18days post fertilization. In response to LPS challenge, C6 was peculiarly expressed: it was significantly decreased before hatching, followed by a significant up-regulation post hatching. Taken together, these suggest that the complement operating via the AP is already competent by responding to challenge with LPS in the hatched larvae of Danio rerio.