Toxoplasma gondii is a persistent protozoan parasite capable of infecting almost any warm-blooded vertebrates. SAG1 (p30) is the prototypic member of a superfamily of surface antigens called SRS (SAG1-related sequence). It constitutes the most abundant and predominant antigen. In this paper the primary structure of mature SAG1 gene of an Indonesian T. gondii isolate is described and sequence comparison is made with published sequence data of 7 other strains or isolates. Sequence comparison indicated that SAG1 is highly conserved through evolution and despite parasite spreading world-wide. Sequences may be divided into two major families, independent of the strain/isolate geographic origin. Variations were mainly localized at the C-terminal half or domain 2 and some clustered in restricted areas. Sequence comparison allowed us to define the Indonesian isolate as genuine virulent RH strain. A phylogenetic tree of Toxoplasma strains/isolates was constructed based on SAG1.