Diets of salmonines in Lake Michigan have been dominated by alewives Alosa pseudoharengus since the 1960s, and information on alewife population dynamics is critical to the management of salmonine fisheries. We monitored alewife size at age and condition (K) at several different locations in Lake Michigan during fall 1984–2001. Alewives were aged by enumerating annuli on otoliths. The results indicated that alewife length at age did not trend upward or downward between 1984 and the late 1990s but decreased from the late 1990s to 2001. Alewife weight at age was relatively constant between 1984 and the mid-1990s but decreased from the mid-1990s to 2001. Mean condition for a given alewife age was, on average, 13.7% higher during 1984–1994 than during 1995–2001. This decline in alewife condition was not a density-dependent response by the alewife population because alewife abundance trended neither upward nor downward during 1984–2001. The decline in alewife condition was possibly due to the lakewide decrease in the abundance of Diporeia spp. during the 1990s. Apparently, the availability of the large-bodied invertebrates Diporeia and Mysis spp. was an important regulator of adult alewife growth because alewives attained a substantially larger size in Lake Michigan, where these invertebrates were relatively important constituents of the adult alewife diet, than in Lake Ontario, where these invertebrates were not readily eaten by adult alewives. For age-2 or older females, mean length was 2–9 mm greater than for males. Alewife size at age and condition were slightly higher on the eastern side of Lake Michigan than on the western side.