An experiment was conducted on the post-mortem dissolution of calcareous foraminifera in situ at a fixed station(water depth 2.4 m) in Lake Saroma, northern Japan on the specimens of Ammonia beccarii forma 1, which is a typical taxon of the brackish water and bay environment. The study revealed that the species undergone significant dissolution within one month's time, indicating thereby that the foraminiferal calcareous material may be rapidly dissolved in brackish and bay setting.