Educators often compare quantitative scores on motivational traits across time (e.g., pre- and post-semester), yet few studies have examined the longitudinal measurement equivalence of such traits or described typical trajectories. Our research explored the longitudinal measurement of individual interest in two cohorts of an introductory animal sciences course across four measurement occasions during a 16-week semester. First, we modified an existing individual interest scale and validated it within our population (CFI = 1.00, SRMR = 0.02, RMSEA = 0.05). Second, we established partial scalar invariance across measurement occasions through nested model comparisons. Third, we described the trajectory of individual interest with latent growth curve models (LGCM). Individual interest started high for both cohorts (intercepts = 65.02, 61.06 on a scale from 0 [low] to 70 [high] for Fall 2018 and Spring 2019, respectively). Individual interest followed a curvilinear pattern in Fall 2018; however, no significant shape trends described Spring 2019 data. Overall, our results show that individual interest can be measured equivalently across a semester; however, it follows heterogenous trajectories. Further research is needed to improve the sensitivity of individual interest scales within high-interest populations and relate heterogenous interest trajectories to classroom experiences.