The flagellate Herpetomonas roitmani is a symbiont-bearing trypanosomatid that spontaneously differentiates from promastigote to para- and opisthomastigote forms when maintained in axenic culture medium. Thus, after cultivation for 72 h at 28 degrees C, 37% of the total number of cells are in the opisthomastigote form. In the present study, light microscopy observations of Giemsastained H. roitmani cells demonstrated that in early cultures (12 h at 28 degrees C) the percentage of opisthomastigotes was markedly high (about 98%). Furthermore, proliferative opisthomastigote forms (dividing cells with the kinetoplast posteriorly located relative to the nucleus) were frequently seen in these cultures. The latter observation was confirmed by analysis of routinely fixed parasites by transmission electron microscopy.