IN their recent review in Nature1 of the papers on an experiment in marine fish cultivation by my colleagues and me2, Cooper and Steven express the view, "that encouragement of fish growth in arms of the open, sea such as the North Sea or the English Channel by the addition of plant nutrients can never be a paying concern". This categorical statement, based on a previous paper by Cooper3, seems to bar once and for all any large-scale application of fertilizers to suitable parts of the sea. Leaving a fuller discussion to a forthcoming paper on fertilizer application in an open sea-loch, may I point out here that Cooper‘s arguments and figures do not, in my opinion, bear out his conclusion. For an assessment of the economics of fertilizing the sea we require figures showing : (a) the return in crop and stock per ton of fertilizer added to land, (b) the return in fish and shell-fish per ton of fertilizer added to the sea. The latter figures unfortunately do not exist. If our experiments have shown anything, it is the desirability of keeping an open mind to a rational approach to the whole problem of increasing the amount of human food growing in the sea, and pursuing research step by step along various lines, not excluding the fertilizing of suitable larger areas. As regards the threatening scarcity of phosphate, should it become necessary to recover it from the sea it may prove possible to extract it from water taken from greater depths of unproductive areas where the concentration of phosphate is high and its availability to plants low.