The transformation of epimastigotes to metacyclic trypomastigotes of the Trypanosoma cruzi clone Dm 28c has been studied in an in vitro system consisting of artificial triatomine urine supplemented with newborn calf serum. The comparison of morphological data with gene expression products, as judged by the proteins synthesized during differentiation, has shown that stage specific gene activation precedes by far the morphological changes of differentiating cells. Immunoprecipitation of differentiating cell antigens with a trypomastigote stage specific antiserum has shown that although the morphological differentiation process takes six days to be completed, epimastigotes start to express the Mr 86 000 and the 78 000 trypomastigote antigens within the first 12 h of induction.