Two sympatric menhaden species, genus Brevoortia, and their natural hybrids from the Indian River, Florida, had fat cycle peaks in autumn and lows in early spring. Fat deposition corresponded to plankton abundance in spring, summer, and fall, and fat depletion was related primarily to the reproductive cycle. A high fat storage capacity in B. tyrannus, compared to B. smithi, appeared to be an adaptation related to energy requirements. Hybrids inherited an intermediate fat storage capacity. Condition factors corresponded to fat content in analyses of seasonal variation and comparisons of populations. Condition factors and fat- body size were used as indicators of fatness. It is surprising that fat cycles of Atlantic Ocean menhaden have not been described, especially since the oil yield of Brevoortia tyrannus is the basis of the fish oil industry in the Atlantic. B. tyrannus is known to range in the Atlantic from Florida to New England, B. smithi from Florida to North Carolina, and hybrid B. smithi X B. tyrannus from Florida to Georgia (Dahlberg, 1966). Menhaden of the Florida coast are the major source of bait for the blue crab fishery of Florida. Menhaden are also used as bait for charter boat fishing and as bait for catfish fishing in Lake Okeechobee. Menhaden are edible but seldom used for food. Commercial fishermen from the Indian River, Florida, distinguish two kinds of menhaden but do not recognize the abundant hybrids as different or intermediate. Discussions with fishermen indicate that they have observed variation in menhaden in fatness, firmness of flesh, amount of body mucus, spotting on the sides, general body color, and caudal fin color -characteristics which distinguish the two species and their hybrids. Fishermen use for bait whatever kinds of menhaden are available, but they have observed that B. tyrannus seems to attract more crabs and fish, while B. smithi is desirable be- cause it does not spoil as fast. The abundant hybrids provide a natural compromise. The objectives of this study are to establish the nature of men- haden fat cycles, to determine the taxonomic and biological signifi- cance of seasonal cycles, and to find a fatness indicator for com- mercial use.