tDetermining of the success of a parasite’s infectiveness in its snail host clearly depends on environmentalconditions. Temperature, one of the most influential factors impinging on metabolism of cold-bloodedanimals, is believed to be an important factor in parasitic infection in snails. In order to elucidate theinfluence of temperature, sex and size of snails on infectivity of Opisthorchis viverrini to its first inter-mediate host, Bithynia siamensis goniomphalos, 960 snails were divided into 2 groups by sex. Each groupwas subdivided by their size into small and medium sub-groups. Each snail was fed with embryonateduterine-eggs of O. viverrini at different temperatures (16–37◦C, 3◦C intervals). Dissections were carriedout 1, 7, 14, 28 and 56 days thereafter and detection of O. viverrini infection was undertaken by PCRusing specific primers. Infection was strongly temperature-dependent, as temperature increases of 1◦Cresulted in increased odds of infection 5.4% (P < 0.01). A temperature of 34◦C gave the highest rate ofinfection of 44.14%. We also found that the odds of infection in small sized snails was 39.8% higher rela-tive to medium sized snails (P < 0.05). Relative to day 1, the decrease in the odds of infection was detectedwhen the day post infection was longer (P < 0.01). Proportion of infection in female was not different tomale significantly.