High Mobility Group B (HMGB) proteins are among the most abundant and ubiquitous chromosomal nonhistone proteins in eukaryotes. HMGBs are involved in DNA transcription, replication, recombination and repair. The vertebrate HMGB1 protein contains 2 homologous DNA binding domains, the HMG boxes A and B. The HMGB proteins are able to recognize and bind preferentially to bent or distorted DNA. They are also able to supercoil and bend DNA, causing structural changes in the nucleic acid. We have recently cloned the Schistosoma mansoni HMGB1 cDNA (SmHMGB1). Schistosoma mansoni HMGB1 is highly conserved within the DNA binding domains (box A and box B) but contains a short acidic tail comprising only 6 amino acid residues, while the vertebrate HMGB1 proteins contain 30 acidic residues in the C-terminus. Vertebrate HMGB proteins have been reported to be posttranslationally modified by acetylation, methylation, ADP-ribosylation, glycosilation and phosphorylation. An in silico prediction of SmHMGB1 phosphorylation sites showed four putative sites for casein kinase II (CKII) (S-167, T-169, S-172 and S-174). Indeed, we were able to show experimentally that SmHMGB1 was specifically phosphorylated in vitro by CKII. The SmHMGB1 constructs that lack the acidic C tail (A domain, B domain and Dacid C tail) were not phosphorylated by CKII, suggesting that the serine residues 172 and 174 may be directly involved in this posttranslational modification. We generated SmHMGB1 mutants by replacing the phosphate-acceptor serine residue located on the Carboxi-terminal tail to alanine, at positions 172, 174 or both. The SmHMGB1 mutants S172A and S174A remained a substrate for CKII phosphorylation, whereas the mutant S172/S174A lost the capacity to be phosphorylated. Additional assays suggested that phosphorylation of SmHMGB1 by CKII might be playing a role on its ability to supercoil DNA.