We have studied the trans-sialidase from insect forms of Trypanosoma cruzi growing in axenic culture. Log phase epimastigotes expressed little or no trans-sialidase activity, and were unable to incorporate exogenous sialic acid. Transsalidase started to be expressed at the late logarithmic phase, with specific activity increasing steadily as the culture reached the stationary phase. Trans-sialidase was purified from the late log phase epimastigote culture, which contained less than 2% of metacyclic forms, yielding a glycoprotein that migrated as a single 90-kDa band in sodium dodecyl sulfate gels. This enzyme features: (1) no reaction with antibodies against the peptide repeats present in the carboxy-terminal of trypomastigote trans-sialidase; (2) positive reaction with antibodies raised against a fragment of trypomastigote trans-sialidase that contains the active site; (3) similar kinetic properties and identical acceptor-donor specificity when compared to the trypomastigote enzyme; and (4) neuraminidase activity in the absence of acceptors. Upon differentiation into metacyclic forms, a trans-sialidase activity containing the carboxy-terminal repeats of the trypomastigote enzyme was released into the medium. These results suggest that epimastigotes express a developmentally regulated trans-sialidase that contains the same catalytic site but lacks the tandem amino acid repeats typical of trypomastigote trans-sialidase.