The objective of this study was to assess seasonal changes in the maturation of adult Cotylaspis insignis recovered from the gill and visceral mass junction of the fat mucket, Lampsilis radiata luteola, over 14 months. Mussels were collected from North Elkhorn Creek, Scott County, Kentucky, U.S.A., between May 2015 and July 2016. Staging of C. insignis (N = 675 worms) was based on the following criteria: Stage 1 = 17–18 peripheral alveoli in ventral sucker and 8 medial alveoli; Stage 2 = 19–20 peripheral alveoli in ventral sucker and 9 medial alveoli; Stage 3 = developing vitellaria present; Stage 4 = eggs present. Seasonal changes in the proportions of these stages were apparent in this study. Recruitment of a new cohort of adult worms by mussels began by December, as evidenced by an increasing proportion of Stage 2 worms; this increase coincided with a steady decrease in Stage 4 worms between November and March. By March and April, developing vitellaria (i.e., Stage 3 worms) became obvious in many C. insignis, and the majority of worms in this new cohort began to engage in egg production (Stage 4 worms) between late May and early June.