Imidacloprid (IMI) may be a viable alternative to carbaryl for controlling burrowing shrimp that destabilize sediments, resulting in poor survival and low yields of Pacific oysters. However, efficacy varies among substrates and laboratory tests indicate the shrimp are relatively insensitive to IMI (24-h LC50's > 12,000 ppb active ingredient [ai]) and 10x greater than those in sediment pore water following application of 4x the desired application rate. These results contrast sharply with observed efficacy in the field, with significant mortality occurring 72-96 h post application in sandy substrate at 0.5 lbs ai/ac. Reasons are not known, but an understanding of the factors governing efficacy in the field may improve control within muddy substrates and those with eelgrass. We exposed adult N. californiensis in seawater to IMI (Nuprid® 2F) simulating concentrations reported in sandy substrate following application of 0.5 lbs ai per acre. Concentrations were the means from a pore-water fate curve: 0-6 h = 150 ppb ai, 6-24 h (24), 24-48 h (10), 48-72 h (6), and 72-96 h (4.3) plus negative controls with 10 shrimp at each time interval (5 IMI, 5 controls), a loading of 0.31 L/g, and water exchanges at the end of each time period. Water quality and overt effects were monitored. At the end of each exposure, the ability of shrimp to burrow in sandy substrate was evaluated. All IMI-exposed shrimp survived, were overtly affected, and none burrowed. In contrast, all controls appeared normal and 96% exhibited burrowing behavior. Results suggest applications of IMI (0.5 lbs ai/ac) will not directly kill shrimp, exposed shrimp may not be able to maintain their burrows, and death may result from burrow collapse. Factors that maintain burrow integrity (mud/organic matter, eelgrass roots) may explain differences in observed efficacy. Laboratory tests with shrimp exposed to IMI within burrows are needed.