An analysis is made of the mysids which occurred in a series of bottom stramin net plankton hauls made at three stations in the Plymouth area in 1927–8.

There were two marked maxima in numbers of mysids, a summer one in July and August, and a winter one in December and January.

The summer maximum was due entirely to Leptomysis gracilis.

The winter maximum was due mainly to Schistomysis ornata and Anchialina agilis, but the two species of Gastrosaccus, and Neomysis longicornis also contributed to a lesser degree.

The summer maximum was composed mainly of adult males and breeding females of Leptomysis gracilis which appear to become markedly planktonic in habit at the breeding season though keeping to the deeper layers. Immature specimens were present but the majority were probably higher up in the water (mid-water).

There is a suggestion that a similar planktonic habit is to be found in Schistomysis ornata and Anchialina agilis during their breeding period in the winter, but it is not nearly so marked as in Leptomysis gracilis.

The winter maximum is mainly due to large numbers of immature forms and it is suggested that environmental factors, probably temperature, cause a concentration of immature stages in the bottom water during winter. At other times of the year these immature forms are distributed over a much greater depth range of water.

Idle swimming movements just over the bottom are a normal habit of bottom living mysids.

The collection provides evidence of three types of movements in mysids: (i) idle swimming just over the bottom, (ii) definite and marked movements into the plankton in the adult condition during the breeding season, and (iii) movements of the immature forms to greater or lesser depths according to environmental conditions, probably temperature.