A study of age and growth rates of four species of fish taken from the Missouri River between Holter Dam and Cascade, Montana, was made during the summers of 1948 and 1949. Four hundred and seventy-eight rainbow trout, 127 brown trout, 87 longnose suckers, and 223 western white suckers were examined. Age groups of rainbow trout ranged from I to VII; brown trout, I to IX; longnose suckers, III to VIII; western white suckers, I to VIII. Age group II was most predominant in rainbow and brown trout, group VI in longnose suckers, and group III in western white suckers. Greatest growth was attained during the second year in rainbow and brown trout, and during the first and third years for longnose and western white suckers, respectively. Trout growth is more rapid than sucker growth. The majority of trout were represented in age groups I-III, while suckers were predominantly represented in age groups IV-VIII. Coefficient of condition for 481 rainbow trout was 37.2 and for 117 brown trout was 34.6.