This key to British rotifera is intended to be used by those unfamiliar with their structure and habitats, and is based, as far as possible, on those morphological characters which are most easily observed. The main character which has formerly been used to separate most families is the structure of the jaws. As this character is not easy to observe in many rotifers, it is used as little as possible in the present key. There is no separate key to families; the key gives identification to genera and then to species, which are grouped in families. Most rotifers encountered are females, reproducing parthenogenetically for most of their season of occurrence. Characters given in the main keys and descriptions of species apply mostly to females, but notes on the males are included. A key to genera of males is given separately.