This study investigated effects of various seasonal factors on buoyancy attained by fathead minnows, Pimephales promelas. Fish were taken from the field to the laboratory at regular intervals (when possible) over 2 yr, and buoyancy attained in either still water or current was measured. Buoyancy attained in still water and in current (maximum tolerable velocity) varied seasonally when fish were held and tested at a natural photoperiod and temperature of capture. Seasonal variation still occurred when fish were held and tested at a natural photoperiod but at 15 °C, and for those in current, at a velocity of 35 cm∙s−1. There was no seasonal variation in buoyancy attained by fish held in the laboratory under constant environmental conditions. Effects of photoperiod, sex, gonad development, estimated fat content, and condition of the fish on buoyancy were determined. Fish exposed to a photoperiod of 12 h illumination were less buoyant in still water and current than those held under longer or shorter photoperiods. There was no significant difference in buoyancy attained between females and males. However, buoyancy decreased significantly, especially in current, as gonad development increased. Buoyancy increased significantly in still water and in current with days of starvation. As coefficient of condition decreased, buoyancy increased significantly in current but not in still water. Estimated fat content, determined by a densitometric method, had no significant effect on buoyancy. Seasonal variation occurs in the ability to attain a particular buoyancy and this is influenced by a number of potentially interacting factors that include water temperature and velocity, photoperiod, gonad development, age, and condition of the fish.Key words: fathead minnow, Pimephales promelas, buoyancy, seasonal variation, photoperiod, condition of fish, gonad development