The crustacean zooplankton of these large lochs is described from a series of vertical net hauls taken there during 1977–78. A total of 11 planktonic species of Crustacea was found — six of them common to all the lochs. There were sufficient differences in the occurrence of the other five species for it to be possible to identify the community of each loch from a single haul during summer. The basic population dynamics of all species are described and compared among these lochs. The lochs themselves are compared on the basis of a number of features of the crustacean zooplankton — especially the total number of species and individuals, and the mean and maximum numbers of each species. It was found that Loch Lomond exhibited an overall dominance in terms of the criteria considered, and that Loch Morar was lowest in almost all respects. The significance of these findings is discussed in relation to other factors.