Anti-lipopolysaccharide factors have been described in arthropods, including penaeid shrimp. In this study, we characterize six antilipopolysaccharide-coding clones isolated from a cDNA library from L. vannamei hemocytes. Three different isoforms (named ALFLv1, ALFLv2 and ALFLv3) are observed, based on three nucleotide differences that produce three changes in amino acid sequence (Val11Ala, Val12Ala, Arg83Lys). Other single differences in nucleotide sequences were also noted, but they do not change the translated product. Considering that the signal peptide cleavage site occurs between Ala25 and Gln26, the substitutions Val11Ala and Val12Ala are not present in the mature protein. The mRNA steady state levels of ALFLv3, but not for ALFLv1 or ALFLv2, were up regulated by Vibrio alginolyticus inoculation. Thus, the differences among penaeid ALFs seem to be associated with the Arg83Lys substitution and its expression under bacterial inoculation.