The strength of precopulatory sexual selection may vary within a breeding season depending on fluc-tuations in the social environment faced by active individuals. Here, we investigated within-season variations in precopulatory sexual selection in the river lamprey, Lampetra fluviatilis. We determined individual lifetime mating success in a group of 20 females and 15 males through constant video recording in an experimental aquarium. Among each sex, we calculated the potential strength of sexual selection and selection on identified traits (body size and temporal spawning patterns) at two different timescales: a fine-grained scale in which we considered mating events at daily time steps and a global scale in which we pooled all mating events that occurred during the breeding season. We found a predictable change in the operational sex ratio that corresponded with a change in the opportunity for sexual selection in males. Selection on both male and female body size was stronger when active competitors were larger. In addition, the timing (onset and span) of both the male's and the female's reproductive activity affected the number of matings performed over the season with a strength dependent on individual body size. Overall, our study documented that river lampreys experience within-season changes in precopulatory sexual selection. Such temporal dynamics are important to consider, and we advocate for longitudinal observation of mating behaviour to complement traditional time-integrated estimation of mating success based on genetic parentage analysis or cross-sectional analysis.