Alewives Alosa pseudoharengus collected in southeastern Lake Michigan from October 1979 to October 1981 were analyzed for caloric content, lipid, and lean dry (lipid-free) weight. The relative seasonal change in energy density (kcal/g wet weight) was similar in adults and immature fish, although immatures generally had lower caloric equivalents. Seasonal lows in caloric equivalents occurred in April for immature fish in their first year (1.03 kcal/g) and in June for adults (1.30 kcal/g), whereas maxima were found in October for all age groups (2.30 kcal/g). Somatic energy losses during winter were 27% for adult females and 23% for males; spawning-related losses were higher (37% and 45%, respectively). Percent lipid followed the same seasonal trends as caloric equivalents with maxima in the fall (17% wet weight) and minima in late spring-summer (3–5%). Approximately 80% of gonad growth and maturation took place from April to late May when feeding was at a minimum. Energy requirements for gonad growth apparently came mainly from body lipid, although body protein also was used. Most body growth occurred in the fall. Seasonal increases and declines in growth were related to food availability, temperature, and spawning. Annual die-offs correspond with seasonal energy lows, indicating that mortalities may be induced by insufficient feeding the previous fall.