Technical Abstract: The effect of fipronil treated sand on digging behavior and mortalities of red imported fire ant workers, Solenopsis invicta Buren, was examined in the laboratory. No-choice digging bioassays where fipronil treated sand was the only available digging substrate, were conducted on two colonies at fipronil concentrations of 0.00, 0.05, 0.10, 0.50, 1.00, 1.50, and 2.00 ppm. Workers dug into the fipronil treated sand in all cases, even at 2.0 ppm level which caused 100% mortalities in acute toxicity tests for both colonies. At 1.5 and 2.0 ppm, workers from the less sensitive colony had significantly higher mortalities than those from the more sensitive colony, which might be explained by the significantly higher digging activity of the less sensitive colony. In two-choice digging bioassays where untreated sand was also available, workers dug into the fipronil treated sand in 29 of 30 cases, even at 10.0 ppm level. At 1.0 and 10.0 ppm, mortalities were positively correlated to digging effort in treated sand; however, such correlation was significant only at 1.0 ppm level. This indicates that digging did affect mortality; however, such effect is concentration dependent.