Brackish marshes resulting from embankments are a rare habitat in Europe and for which there is little knowledge of the entomofauna. There is an urgent need to document the insect diversity and its unique properties in the light of ongoing habitat losses. The current study focuses on snail-killing flies in the remaining Belgian brackish marshes, in particular the role of soil salinity in determining their diversity and how it compares with that in freshwater marshes. Snail-killing fly communities in brackish marshland were relatively species poor and clearly different from those in freshwater, yet species richness and abundance increased with decreasing soil salinity. We recorded no habitat-specific species, but Tetanocera arrogans was more abundant in brackish marshes. Malaise traps proved to be a successful and repeatable method of sampling sciomyzid communities, the species composition of which was strongly site-specific. We conclude that extant Belgian brackish marshes have poor to moderately rich snail-killing fly communities that lack unique species. Richness and abundance, however, increased as soil salinity decreased.