The importance of fraction knowledge to later mathematics achievement, along with U.S. students' poor knowledge of fraction concepts and procedures, has prompted research on the development of fraction learning. In the present study, participants' (N = 536) development of fraction magnitude understanding and fraction arithmetic skills was assessed over 4 time points between 4th and 6th grades. Latent state-trait modeling was used to examine codevelopment of these 2 areas of fraction knowledge. Fraction arithmetic skill predicted later fraction magnitude understanding, and conversely, fraction magnitude understanding predicted later fraction arithmetic skill. The results are consistent with a bidirectional model of the development of fraction concepts and procedures, in which knowledge of one type facilitates learning of the other type. However, transfer in both directions between fraction arithmetic skill and fraction magnitude understanding was more likely to occur later in the development of fraction knowledge, after fraction arithmetic with unlike denominators had been taught in school (during 5th grade in the current sample). Furthermore, the effects of previous knowledge of the other type were small and not nearly as substantial as the effects of previous knowledge on later knowledge of the same type. Findings suggest a need for instruction to link fraction magnitude understanding to fraction arithmetic skill and vice versa.