The mobility of imidacloprid [1-(6-chloro-3-pyridinyl)methyl]-N-nitro-2-imidazolidinimine] from alginate-based controlled-release (CR) formulations was investigated in two different soil pro®les. In one, a layered bed system simulating the typical arrangement under a plastic greenhouse, which is composed of sand, peat, amended soil and native soil, was used. In the other, the layer containing amended soil was used in order to determine the mobility of the insecticide in a soil system with a low content of organic matter and a high content of clay. Two CR formulations based on sodium alginate (1.87% wt/wt), imidacloprid (1.21%), natural or acid-treated bentonite (3.28%), and water (93.64%) were compared to technical grade imidacloprid. The use of alginate CR formulations produced less vertical mobility of the active ingredient as compared to the technical product. With the technical grade product treatment, the total amount of imidacloprid leached from columns packed with amended soil was 82.3% of that applied, whereas for the alginate-based CR formulations containing natural or acid-treated bentonite, the leached percentages were 44.7% and 37.1%, respectively. In the column experiments simulating the layered bed system, no insecticide was found in the leachate when the alginate-based CR formulations containing natural bentonite were used. However, 3% of the applied imidacloprid appeared when the treatment was carried out with technical grade material. Sorption-desorption capacities of the various soil layers for imidacloprid molecules were also calculated using batch experiments.