Human foamy virus (HFV) integrase mediates integration of viral c-DNA into cellular DNA. In this process, HFV integrase recognizes its own viral DNA specifically and catalyzes insertion of viral c-DNA. In order to study catalytic domains and residues, three deletion mutants and two point mutants of HFV integrase were constructed and analyzed with respect to enzymatic activities. The C-terminal deletion mutant showed decreased enzymatic activities while the N-terminal deletion mutant lost the activities completely, indicating that the N-terminal domain is more important than the C-terminal domain in enzymatic reaction. The point mutants, in which an aspartic acid at the 164th position or a glutamic acid at the 200th position of the HFV integrase protein was changed to an alanine, lost the enzymatic activities completely. However, they were well complemented with other defective deletion mutants to recover enzymatic activities partially. Therefore, these results suggest that the aspartic acid and glutamic acid at the respective 164th and 200th positions are catalytic residues for enzymatic reaction.