The flagella of several Trypanosomatidae (Trypanosoma cruzi, Herpetomonas samuelpessoai, Leptomonas samueli, Herpetomonas megaseliae, and Crithidia harmosa) were studied. Besides the axoneme, they have a filamentous, latticelike structure, the paraxial or paraflagellar rod. This structure was not observed in the flagellum of the intracellular spheromastigote form of Trypanosoma cruzi, but it appeared when the transformation of epimastigote and trypomastigote stages occured. Cross sections of the flagella show that the paraxial structure maintains a fixed position relative to the axonemal microtubules, being localized in a region comprised between doublets 4 and 7. Flagella of T. cruzi and H. samuelpessoai were isolated using the detergent lubrox PX. Most of them did not have a flagellar membrane. However, the paraxial structure remained associated with the axoneme. By negative staining, short projections were seen connecting the paraxial structure to the axonemal microtubules. The paraxial structure did not stain with phosphotungstic acid as occurs with the peripheral doublet microtubules, but it is formed by microfilaments longitudinally oriented in relation to the axoneme crossed in two directions by oblique filaments which make an angle of 45 degrees with the longitudinal ones.