Adult males of Gammarus pulex may reach a total length of 21 millimetres (0.83 in), while females only grow to 14 mm (0.55 in).

[2] Gammarus pulex is found across most of Europe from the Volga drainage in the east to the British Isles in the west.

[3][4] It is absent from Norway, parts of Scotland, and Ireland,[3] although it was introduced to Lough Neagh in the 1950s, where it is replacing the native Gammarus duebeni.

[5] Gammarus pulex was one of the species included in the 10th edition of Carl Linnaeus' Systema Naturae, which marks the starting point for zoological nomenclature, in 1758.

The application of the name was settled in 1970 by the redescription of the species and the selection of a neotype from a stream on the Swedish island of Öland.