The reproductive biology of the crayfish Orconectes luteus (Creaser) from the Jacks Fork River, Missouri, was studied. Data on reproduction, fecundity and growth were obtained from monthly or semimonthly collections over 27 mo. Females oviposited in March or April, depending on water temperature, and the young became independent by May or June. Seasonal growth and molting began when water temperatures exceeded about 9 C in spring and ceased when water temperatures dropped below 8 C in fall. About one-third of the male and female crayfish became sexually mature their first year of life. Adult males began molting to the reproductive state (Form I) in September and October and returned to the nonreproductive state (Form II) in April and May when the females were carrying eggs and young. Newly hatched young went through two molts before becoming independent from the female in the third instar. A significant positive relation was found between crayfish size and number of ovarian eggs, pleopodal eggs and size of pleopodal eggs. Significantly greater numbers of ovarian vs. pleopodal eggs were found at both sites.