An environmental fate study was conducted in a citrus orchard plot in València (Spain) in the fall of 1993. Dissipation and distribution of atrazine, simazine, chlorpyrifos and tetradifon residues following their controlled addition for agricultural purposes in a mediterranean red soil (Luvic Calcisol, Rhodoxeralf) were evaluated. During a two-month period, the amounts of applied pesticides in different soil layers (0-0.05, 0.05-0.22, 0.22-0.42, and 0.42-0.52 m) were monitored. In addition, information on soils, weather and agricultural practice were collected. Degradation half-lives were calculated, assuming zero-order kinetics: 11 days for atrazine, 12 days for simazine, 10 days for chlorpyrifos, and 18 for tetradifon. The distribution through the soil profile shows that the pesticide concentrations were always highest in the upper layer (0-0.05 m) of soil, and that atrazine was the most mobile of all the four pesticides investigated.