Studies were based primarily on 3,097 bloaters collected in experimental gill nets and bottom trawls fished in 1958-65 in the Apostle Islands region of Lake Superior. The average size of bloaters increased considerably during the period. The percentage longer than 8.9 inches increased from 45% in 1959 to 99% in 1965. Calculated lengths were without exception higher in 1962-65 than in 1958-61. Growth in length and weight showed nearly steady improvement from 1951 to 1964. The species composition of chubs has apparently changed considerably over the past 40 years. The bloater has replaced the shortjaw cisco (C. zenithicus) as the principal species in commercial landings of chubs in Wisconsin. Annulus formation of bloaters extended from mid-May to August. Growth was slow during the spring and early summer but increased sharply in August. Age-group VI was dominant in the samples. Fluctuations in year-class strength were slight. Females outnumbered males in all age groups above III. The sex composition varied according to season of capture. Some bloaters matured at age II and all fish older than III were mature. The shortest length of mature bloaters was 6.0 inches for males and 7.0 inches for females; all fish longer than 8.4 inches were mature. Apostle Island bloaters spawn principally in February and March. The average number of eggs produced by 20 females, 8.4 to 11.7 inches long, was 6,533. Crustaceans were the most common food.