In order to control saprolegniosis in Prussian carp (Carassius gibelio (Bloch, 1782) eggs, it is important to screen herb extracts as potential anti-Saprolegnia drugs in Prussian carp hatcheries. For this purpose, an oomycete water mould (strain SC) isolated from Prussian carp [Carassius gibelio (Bloch, 1782)] eggs suffering from saprolegniosis was characterised morphologically as well as from ITS rDNA sequence data. Initially identified as a Saprolegnia sp. based on its morphological features, the constructed phylogenetic tree using the neighbour joining method further indicated that the SC strain was closely related to Saprolegnia australis R. F. Elliott 1968 strain VI05733 (GenBank accession no. HE798564), and which could form biofilm communities as virulence factors. In addition, aqueous extracts from forty Chinese herbs were screened as possible anti-Saprolegnia agents. Among them, a 1 g ml−1 extract from Radix sanguisorbae was the most efficacious anti-Saprolegnia agent, indicated by the minimum inhibitory concentration that was as low as 256 mg L−1. Relative survival of 73 and 88% was obtained against the SC strain in fish eggs at concentrations of 256 and 1280 mg L−1, respectively. This is the first known report of Saprolegnia australis R. F. Elliott 1968 infection in C. gibelio (Bloch, 1782) eggs involving the screening of R. sanguisorbae extracts as potential anti-Saprolegnia agents.