The effect of thyroid activity on growth of green sunfish and the influence of photoperiod on thyroid activity were investigated. Fish of three thyroidal levels were held in light‐tight aquaria at constant temperatures under four photoperiods (8‐hour constant, 16‐hour constant, variable increasing 8‐16 hours, and variable decreasing 16‐8 hours). Differences in growth of fish given injections of thyroxine (hyperthyroid), radioiodine (hypothyroid), and saline solution (control) demonstrated that the hyperthyroid fish attained the greatest growth and hypothyroid fish the least growth. Increased food consumption and increased efficiency of food conversion were generally associated with greater thyroid activity and/or greater availability of thyroxine. Using rate of loss of radioiodine from the head region as an index of thyroid activity, it was found that greater thyroid activity was associated with short photoperiods and low thyroid activity with long photoperiods.