Crustacean plankton populations were compared for the 1950’s and 1975 in eight deep lakes of northwestern lower Michigan. Special attention was focused on four cold stenothermic species, Mysis relicta, Pontoporeia affinis, Limnocalanus macrurus, and Senecella calanoides. These crustaceans are considered glacial relicts and are found in many oligotrophic inland lakes of formerly glaciated regions. In 1975, the full complement of crustaceans observed in the 1950’s were still present in five of the lakes. However, Mysis was absent in Lake Bellaire, Limnocalanus was not collected in Crystal Lake, and Pontoporeia was not found in Glen Lake. These organisms are important food sources for cold-water fish species, especially salmonids. Glacial relict crustaceans appear particularly susceptible to environmental disturbances, especially water quality degradation and predation from introduced fish species.