A number of excursions to the Celtic Sea have been made, first, to obtain for Dr D. P. Wilson water as contrasted as possible with that in the English Channel near Plymouth, and secondly to obtain answers to hydrographical queries. This paper presents some of the chemical findings which may prove to be relevant to Wilson's investigations on the development of the young planktonic stages of invertebrates. Since not all sea water in the Celtic Sea was likely to be equally suitable for his work, some means was needed for quickly assessing waters on the spot. There are a number of objections to the use of phosphate or other chemical analyses on board so that examination of plankton hauls was chosen. Suitable water was selected on the basis of the bulk and variety of zooplankton and the presence of species known to indicate ‘elegans’ water. The 2 m. stramin ring-trawl, fished for the standard half-hour oblique haul, was used.