Spontaneous metacyclogenesis in culture of two Paraguayan T. cruzi strains has been studied. T. cruzi JAG and RF, two stains isolated from patients at IICS, maintained through successive cultured in LIT medium and the reference strain, y, were cultured in modified Grace medium (isola et. al. 1986), for the purpose of evaluating their ability to develop into the metacyclic trypomastigote from in vitro. T. cruzi y showed a cosiderable higher yield of metacyclic froms (55 percent), as compared with the JAG and the RF strains did not show a significant difference in terms of their metacyclogenic power. These results will be useful in the characterization of T. cruzi strains isolated in Paraguay