Since the early 1960s studies on zooplankton biomass and species composition have been carried out on standard transects during late May and early June in the waters north of Iceland. In the early 1970s the studies were extended to also cover the waters south, west, and east of the island. The present article summarizes these long-term investigations in spring for 62 stations on 9 transects. Calanus finmarchicus usually constituted about 50–80% of the number of animals in the zooplankton samples. Other species/groups of considerable importance in certain areas were C. glacialis, C. hyperboreus, Metridia longa, euphausiid larvae, and cirripede larvae. The horizontal distribution of the zooplankton biomass from shallow to deep water and in different areas was usually relatively uniform (2–4 g m−2), except for the frontal area between coastal and Atlantic Water south of Iceland (ca. 10 g m−2) and in the waters off the north-east coast beyond the shelf in the Arctic East Icelandic Current (9–11 g m−2). North-east of Iceland this is explained by both differences in the state of development and in the composition of the zooplankton. The difference between the highest and lowest biomass on the different transects was 7–24-fold. Principal component analysis (PCA) for the transects in the Atlantic Water south of Iceland revealed maxima in biomass during the mid-1970s, 1980s, and an upward trend since the early 1990s. Lows were observed in the Atlantic Water during the late 1970s and late 1980s. In the transitional waters north-west and north of Iceland (mixed Atlantic/Arctic) PCA demonstrated maxima in the early 1960s, late 1970s and a clear upward trend since the early 1990s. During the minima observed in the intervening periods, slight upward shifts occurred during 1971–1973 and 1984–1986. For the Arctic waters of the north-east and east coasts the PCA showed maxima during the mid-1970s, 1980s, an upward trend since the early 1990s and minima during the late 1970s and late 1980s. Thus, for both the Atlantic and Arctic Waters PCA reveals a period of approximately 10 years between maxima in biomass, while in the transitional waters (mixed Atlantic/Arctic) this period appears to be 16 years. The abundance of 0-group cod in Icelandic waters during 1974–1992 suggests strong interactions with the zooplankton on their spawning grounds and larval drift route to the south and west of Iceland.