Trypanosoma cruzi, the agent of Chagas' disease, exhibits considerable biological variability. Moreover, it has been postulated that populations of this protozoan are subdivided into natural clones, which can be separated from each other by considerable levels of evolutionary divergence. The authors have proposed that this long-term clonal evolution may have a profound impact on Trypanosoma cruzi biological diversity. In order to test this hypothesis, 16 T. cruzi stocks representing 3 major clonal genotypes of the parasite were analysed for 8 different in vitro biological parameters. The overall results show a strong statistical linkage between genetic and biological differences. This is in agreement with the working hypothesis, although a notable biological variability is observable among the stocks of each of the 3 major clonal genotypes. The authors propose that T cruzi genetic variability must be taken into account in any applied study dealing with this parasite.