Purely qualitative investigations of the plankton of an area cannot provide sufficient i nformation for the full understanding of the seasonal changes that oocur nor of the interrelation of the various planktonic plants and animals. Quantitative data must be obtained before these can be properly described. Except for the qualitative records by Scott (1906, 19(7) and Hiddell (1914) the plankton of waters off the north ooast of ales, and. of the l-ienai straits in partioular, has tend d to be neglected. The UlluSual hydrographic conditions of the enai .;)traits, which connect areas whose fauna and flora show considerable differences (Crisp & Knight-Jones, 1953) , sUggested such studies would be rewarding. In describing the various populations the policy in the present work has been to give the details of the most numerous speoies of both plants and animals individually and to summarize the others in various groups . This simplifieD the description and discussion of the dynamics of the population, which would be difficult to follo if all species were described in detail. At an early stage in the investigations it became obvious that the plankton of these coastal waters differed considerably from that of the Irish es in general , which has been described by Willi8I:lson (1 956) . In particular tho copepods Oi thonina .!!!!l!. (Giesbrecht) and terniIm acutifrons , (Dana) were prominent species in the Menai Straits. Euterpina acutifrons was found to have several points of interest. Firstly, dioorphiam occurs in the malo; this has received considerable attention and has been found to be of so significonce in the bi'oloey of the speoies . Secondly, Euterpina is pelagic, while moat other harpacticoid copepoda are benthio or littoral . Investigations of its a.evelop ant ( hich was inadequately described by Tesch (1915)) .in culture allowed a consideration of the adaptive modifications of Euterpina to a pelagic existence, of its rate of grouth and development, and of the developmental aspects of dimorphism in the male. The latter has been a controversial subject in previous stuaies of copepod development (~ewell , 1912, 1929, 1940; Gurney, 1929; Coker, 1934) . Thirdly, ";uterpina has a world-wide distribution, mainly c<""rc ... ~ " in the warmer seas. Consideration of this led to investigations of some aspects of the relationship between the environment and th devslopm nt of the species and also to a comparison of ita breeding season in the wat rs round Anglesey with those reported in other latitudes . The present work also includes studies of the larval d velopment of Qithonina.nena another warm water form. Despite its being a very common plankton animal, the larval. development of this species has tended to be neglected. In the work as now presented the studies of these speoies are given a considerable proportion of the total space, since the data provided on their development , growth and breeding illuminate the general study of the plankton which, in turn. forms a background to the detailed studies. The form of preoentation of this thesis ms;y perhaps call for Bome explanation. The secti0D:B into which it is divided have been prepared in the form of separate papers intended for publication. This meane that figures and tables are numbered oonsecutively within each section and not continuously throughout the thesie, and aleo that a separate bibliography will be found at the end of each section" It is hoped that this method of presentation will cause no inconvenience to the reader.