Sterile flies from a prospective mass-production strain of the screwworm,Cochliomyia hominivorax(Coquerel), were tested in the field on two occasions between August and November 1981 in the Pacific coastal plain of Chiapas, Mexico. In Test I, flies were released over a 30-by-120-km area where all nine sentinel sheep sampling sites were located 5 to 23 km, including 5 sites<10 km, from the nearest perimeter. The maximum egg mass sterility rate (25%) was obtained by the third of 6 weeks of sterile fly releases. In Test II, flies were released over a 40-by-80-km area where all eight sentinel sheep pens were located 18.5 to 34.5 km from the nearest perimeter. For all pens, the maximum egg mass sterility rate (38%)was obtained by the fourth week. Egg mass sterility rates at sites considered well-insulated from the perimeter were statistically higher than sterility rates at sites considered poorly insulated. In Test II, distance from the sentinel sheep sampling sites to the nearest perimeter of the fly release area and egg mass sterility rate were positively correlated, and a maximum sterility rate of 60% was recorded. This relationship was attributed, at least in part, to the movement of native screwworm females into the test area from untreated areas.