Larval stages of trematodes obtained from the freshwater snail Melanoides tuberculata (Cerithioidea, Thiaridae) as intermediate host were studied by using cercarial emergence and crushing snails. Between December 2004 and September 2009 snails from one hundred twenty locations in Thailand were collected every two months for one year at each sampling site. Counts per unit of time method was used in this study, and the samples of snails were collected every 10 minutes per sampling by five collectors. The cercarial stages were examined using shedding and crushing methods. The infection rate was found to be 18.79%, i.e. 6,019 animals infected in a total of 32,026. Nine different types in eighteen species of cercariae were categorized, viz. are (1) Parapleurophocercous

Given the importance in tropical diseases caused by food-borne trematodes infecting humans, it is essential to understand the taxonomy and distribution of its first intermediate hosts, which are in Southeast Asia freshwater Cerithioidean gastropods such as the widely distributed Melanoides tuberculata. Next to the occurrence of this type species in Thailand there is a second, congeneric thiarid neglected and, therefore, remained enigmatic to date. Described as "Melania" jugicostis by Hanley & Theobald (1876) it was originally reported and only known from the type locality, "Tenasserim River" (i.e. Tanintharyi River) of Southeast Burma (today Myanmar), but later recorded also for Thailand based on shell morphology. In evaluating the status of this taxon, we here report on its geographical occurrence ranging from the Kanchanaburi Province to the southern most provinces in Thailand (anticipating that it might extend further south on the Malay Peninsula and even into Sumatra). Based on the more detailed study of material from five locations we provide conchological and anatomical evidence for the comparison with other thiarids, in particular the closely related Melanoides tuberculata and Pseudoplotia scabra. Interestingly, with respect to its reproductive mode we found in one population of M. jugicostis from Erawan waterfall, Kanchanaburi Province, the lack of any sign of a subhaemocoelic brood pouch otherwise typical for viviparous thiarids. This is discussed in context of climatic factors (e.g. seasonal changing precipitation), but anticipated to be caused by an unusually high infection rate of human intestinal trematodes. These parasites having infected 98 of 222 snails studied, resulting in an infection rate of 44.14 %, are assumed to be responsible for the sterility of the females in this particular population, in comparison with considerable lower infection rates in two other populations, viz. Raman waterfall (1.74 % in 5/288) and Klong Chalung (9.46 % in 7/74), respectively.