The promastigote form of Herpetomonas samuelpessoai is modified into a paramastigote-like form during incubation at 28 C in high osmolarity complex media obtained through the addition of various concentrations of KCl, NaCl or glycerol. Morphological changes accompanying this modification include an increase in cell volume during exponential growth, a deep invagination of the cell membrane at the bottom of the flagellar pocket into the cytoplasm, and a prominent bulging at the emergence of the flagellum.