Lobster rearing has been widely carried out in Indonesia, especially to prevent a decrease in stocks in nature due to fishing that does not pay attention to the aspect of resource sustainability. However, in the effort to rear lobsters, there are obstacles, namely necrosis to the tail fins of sand lobsters reared with recirculation aquaculture system (RAS) caused by infectious diseases. This research was carried out at the Field Test Facility of Fish Health and Environmental Assessment Center (BPKIL) Serang in June-November 2021. Sand lobsters had an initial size of 50-100 grams which were reared in 1000-L fiberglass tanks with a density of 25 individuals per tank and reared with RAS. Samples from diseased sand lobster after being reared in RAS for 176 days. Necrosis (red color and erosive) can be seen in the tail fins of sand lobsters reared with the RAS. From the results of laboratory tests, it was found that there were parasitic infections of the types of ciliates, protozoa, and vorticella. The results of further testing at the Microbiology Laboratory found that there was an infestation of vibriosis disease caused by the bacteria Vibrio alginolyticus and Vibrio parahaemolyticus. Diseases caused by parasitic and bacterial attacks do not affect mass death in sand lobsters, even these symptoms can disappear if sand lobsters can replace their skin (moulting) perfectly. Observations also show that the most mortality occurred in sand lobsters with smaller sizes (weight ≤100 grams). This disease infection occurred due to a decrease in the water quality of the sand lobster rearing media in RAS, mainly because of the high organic matter concentration in the water.