To better understand the mechanisms involved in the developmental expression of Trypanosoma cruzi antigens we examined the gene structure and transcription properties of the major shed trypomastigote antigen (SAPA). We report in this paper that SAPA is encoded by a small family of at lest 6 genes which differ mainly in the length of a repeat region made up of tandemly arranged 36-bp repeat units. SAPA genes are located distant from chromosomal telomeres as inferred from their insensitivity to Bal31 nuclease treatment. Furthermore, Northern blot and S1 protection analyses strongly support the fact that most (or all) SAPA genes are transcribed in the infective form of the parasite.