The main features of the life-history of Mermis nigrescens, so far as it is known, are briefly discussed. Some data have been collected on its distribution in the British Isles, and on the seasonal appearance of the egg-laying females. The insect hosts of the worm are discussed, and feeding experiments on various invertebrates are recorded, including the experimental infection of the common earwig (Forflctila auricularia). This insect, which has also been found naturally infested, is regarded as an important host for the species, more especially where grasshoppers are absent. Observations by other workers on the influence of the number of larvae per host on the sex of the developing worms, and on the survival of the host, have been confirmed. Two new species, closely related to M. nigrescens, are described: M. kenyensis and M. tahitiensis, from Africa and Tahiti respectively.