In the spring and summer of 1989, sterile boll weevils, Anthonomus grandis Boheman, were released on two private research farms in Vero Beach, Florida, to study their effect on the native population of boll weevils present there. Egg hatch was reduced to 5% on the Mobay farm where serially planted plots of cotton were destroyed soon after blooming. On the Ciba-Geigy farm, egg hatch in the test plots averaged 32% prior to the formation of bolls 1 in. or more in diameter and 77% in plots where bolls were larger than 1 in. in diameter. It appears that sterile weevils are effective in preventing or significantly lowering the reproductive potential of native weevils entering cotton fields, but if reproduction by the entering weevils is not prevented, the effectiveness of the sterile weevils is significantly diminished. We think that this occurs because mated females lay eggs in a relatively small area, and adults emerging from these "Clumps" of reproductive activity are usually isolated from the sterile weevils being released into the field. Thus, the sterile to native weevil ration in the "clumps" is much lower than that of the field as a whole, and sterile weevils cannot compete effectively traps placed around the perimeter of each farm captured an average of 12 (Mobay) and 37 ( Ciba-Geigy) native weevils per week before and during the period of sterile weevil releases.