A fungal infection occurred in the eggs and larvae of mangrove crab (Scylla serrata) in seed production in Bali, Indonesia. The causative fungus was classified as a member of the genus Lagenidium (Oomycetes, Lagenidiales). After comparison of its biological and physiological characteristics with those of L. callinectes ATCC 24973, a known parasite of various crustaceans, was concluded that the isolate is a new species of Lagenidium, L thermophilum, because of its rapid and thermotolerant growth and unique discharge process. Fungal growth was observed on PYG agar containing 0- 5.0% (w/v) NaCI and 0-2.5% (w/v) KCI. Similar pathogenicity toward the zoeae of swimming crab (Portunus trituberculatus) was demonstrated.