Surface ultrastructure of Parvatrema timondavidi developmental stages was studied using a scanning electron microscope. The metacercariae were collected from the marine clam, Tapes philippinarum, and juvenile and worms adult were recovered at 1, 2, 3, and 7 days after experimental infection of mice. The metacercariae had a large oral sucker and characteristic lateral projections. Around the lip of the oral sucker many type I and type II sensory papillae were observed, and type III papillae were located symmetrically on the medial side of the lateral projection. Numerous type I papillae were grouped around the genital pore. The tegumental spines were distributed over the worm surface except the lip of the sucker and genital pore. The 1-day old worm had a well-developed ventral sucker, with 6 type II sensory papillae on its outer surface and another 6 type I papillae on the inner side, Two small type I papillae were seen on the anterior side of the ventral sucker. The genital pore was and 15 type I papillae were grouped around it. The 2-, 3-, and 7-day worms revealed that as they grew to be adults, the spine tips became multipointed, the genital pore formed a genital atrium, and the cytoplasmic process became well differentiated. In 2- and 3-day worms 10 type II papillae encircling the lip of the oral sucker, and additional 4 papilled at the dorsal side of 4 dorsal type II papillae were a characteristic feature. The distribution pattern of sensory papillae around the oral sucker and genital pore, and 2 type I papillae on the anterior side of the ventra sucker, was so peculiar in P. timondavidi, that they seem to be useful keys for taxonomic differentiation from other gymnophallids.
