The circulation of the northern and north-eastern Celtic Sea, has been studied, using especially observations made in April 1950.

The Celtic Sea in that month was 0·5° C warmer than the average temperature 20 years earlier.

The following distinctive water masses have been recognized (i) Land's End water; (ii) South Wales coastal water; (iii) Carnsore corner current water; (iv) Nymphe Bank water, including a component enriched by up-welling off Waterford; (v) County Cork eddy water.

The South Wales coastal water, though rich in nutrients presumably derived from sewage effluents, was not very productive. A parallel is drawn with the Hyperion sewage scheme of the City of Los Angeles.

The low-salinity water present in the centre of St George's Channel in summer is attributed to flow from the coast of South Wales. In the autumn water is recruited more directly from the Celtic Sea.

The silicate content of the Carnsore corner current was half that in South Wales coastal water on the opposite side of the Channel.

Up-welling of water enriched by breakdown of bottom muds occurred off Waterford.

The drift system in the central Celtic Sea is dominated by the prevailing winds. The drift in April 1950 was a composite affair and it has been possible to dissect some of the components and to show how they interplay.

In about 7° W. long., a mixed water mass sets south from the Nymphe Bank and was identified for more than 100 miles.

A tentative explanation is offered for the distribution of compounds of phosphorus in terms of regeneration from bottom deposits.