Reproduction in female Serrasalmus spilopleura (Characiformes, Characidae) from the Itumbiara reservoir (18°28′ S, 48°36′ W), Paranaiba River, Brazil, was examined with emphasis on the relationship between condition factors and coelomic fat, the annual reproductive cycle and the unusual dynamics of its sexual maturity. The condition factor and coelomic fat index follow a similar pattern along the stages of reproductive cycle. Variations in the condition factor along the reproductive cycle were assigned to variations in the fat contents of the carcass. Serrasalmus spilopleura exhibits characteristics of partial spawning fish and reproduces throughout the year. Sexual maturity occurs at 17.8 cm standard length. Resting and totally spent females were not captured. Females cycle from partially spent to intermediate/advanced maturation without going into the totally spent or resting stages. The initial maturation stage is restricted to fish of intermediate size. Once the initial maturation stage has started the fish remains there for a considerable period of its lifetime before entering in the intermediate maturation stage, an uncommon sexual maturity pattern. The long duration of the initial maturation stage was responsible for the females not returning to the resting stage after spawning.