The Celtic Sea extends from the south of Ireland and the St Georges Channel across the continental shelf, with the Bristol and English Channels as its eastern limits (Fig. 1) (Cooper, 1967). Although various investigations of the physical oceanography (Matthews, 1914; Cooper, 1967) and zooplankton (Russell, 1934, a, b, 1936; Corbin, 1947; and more recently Southward, 1962; Bary, 1963) of this area have been carried out, there is little or no information on seasonal changes in levels of chlorophyll ‘a’ and inorganic nutrients, and on the importance of tidal mixing in determining these distributions. Since the speeds of the tidal streams range from weak (∼ 0.5 knot) in the northern part of the Celtic Sea to strong (∼ 3 knots) around the Scilly Isles and Ushant (Fig. 2), the vertical stability of the water column as well as the duration of the seasonal thermocline (Pingree, 1975) are likely to be important factors in determining spatial and temporal variations of phytoplankton production. In this paper the influence of water-column stability on phytoplankton distributions (in spring, summer and autumn) in the Celtic Sea is described, using data for temperature, salinity, chlorophyll ‘a’ and inorganic nutrients obtained during seven cruises in 1975. An account of the red tide conditions that occurred in late July to the north-west of Ushant has already been published (Pingree, Pugh, HoUigan & Forster, 1975).