Gammarus aequicauda (Martynov 1931), which is considered an important food constituent of fish, shares its breeding places with Aedes detritus, (Haliday) a major pest mosquito in Central Italy coastal marshes. The susceptibility of this crustacean to Altosid was studied in the laboratory, and the LC50 and LC90 values against time, showed a similar trend for the two sexes, while a faster action was shown on young.