The length, age and sex compositions of dead and dying alewives collected in June 1967 at six locations in southern, central, and northern Lake Michigan are compared with those of fish taken in experimental trawls at five locations in April and June 1967. Behavior at the time of death, condition of the body and gonads, stomach contents, and the incidence of Saprolegnia and subcutaneous hemorrhages also are described for alewives in the dieoff. The length distributions of alewives in the dieoff and trawl collections were similar and bimodal. Yearlings made up 25 and 42 percent of the respective collections. The average age of adults (age-group II and older) were nearly identical in the dieoff and trawl collections (3.5 and 3.6 years, respectively). Both collections were dominated by age-groups III and IV. The average lengths of adult alewives in age-groups II to V in the dieoff were smaller than or identical to those in the trawl collections. In all collections females were consistently older (0.4 year) than the males and larger at the greater ages. The sexes were almost equally represented among age-II and older fish in the dieoff. Despite the presence of Saprolegnia and hemorrhages on some fish, alewives in the dieoff appeared robust. Spawning attrition could not have been a major cause of the dieoff because many immature yearlings died and 80 percent of the dead adults were unspawned. The presence of rapidly digestible zooplankton in the stomachs of dead alewives indicated that many fish were feeding just before death. About 20 percent of the alewives in the selected samples of fish from the dieoff were infected by Saprolegnia; twice as many females were infected as males. The fungus was randomly distributed among the size groups. Hemorrhages may have been a symptom or physiological response to the cause of the dieoff because they affected a much higher percentage of the dying alewives (47 percent) than did fungus. Occurrence of the hemorrhages did not differ significantly between the sexes or among the size groups.