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A screening for persistent chlorinated hydrocarbons was carried out in December 1995 in the main coastal lagoons on the Pacific side of Nicaragua, where most of the country’s agriculture and pesticide use has been taking place for decades. Results for a wide range of organochlorine pesticides in lagoon sediments show levels that generally were very low in Estero Real, Estero Padre Ramos, and estuary of San Juan del Sur. For example, total DDTs in these lagoons averaged 4.5 6 3.4 ng g21 dry weight, which may be considered a baseline level for the region. Other compounds such as HCHs, BHC, endosulfan, heptachlor, endrin, toxaphene, and aroclors were present in concentrations even lower, generally below 1 ng g21 dry weight. However, sediments of the Esteros Naranjo–Paso Caballos system at Chinandega district contained pesticide residues in much higher levels, attaining maximum values of 1,420 ng g21 and 270 ng g21 dry weight, respectively, for toxaphene and total DDTs. Other compounds such as aroclors, chlordane, endosulfan, and dieldrin were also present in the sediments of this lagoon system, but in lower concentrations. The very high concentrations of toxaphene and DDTs in this lagoon are a result of the intensive use of these pesticides in cotton growing in the district of Chinandega. Due to the long environmental half-lives of these compounds (t1⁄2 . 10 years in temperate soils), their concentrations in lagoon sediments will likely remain high for years to come. Based on these results, the development of the new shrimp farming activities in the Pacific coastal lagoons should be restricted to selected areas. The intensive use of pesticides in Nicaragua, which for decades has been one of the biggest pesticide importers and users in Central America (Appel 1991; Castillo et al. 1997), is likely to cause severe contamination of aquatic systems. In particular halogenated hydrocarbons, including chlorinated pesticides and industrial chemicals such as the polychlorinated biphenyls (PCBs), are lipophilic toxic compounds that bioaccumulate and transfer in the food chain. Introduced in aquatic environments these chemicals may compromise the health of the ecosystems (Tardiff 1991). This is the case for the coastal lagoons of the Pacific coast of Nicaragua, where most of the country’s agriculture and population have been concentrated. In particular, cotton growing, a pesticide intensive agriculture started in the 1950s, was developed in this region of Nicaragua (Appel 1991). The degradation of these coastal lagoon systems, especially the reduction of mangrove forest and overexploitation of fishery resources, has received focused attention from national authorities. Agrochemical residues are suspected in the degradation of these lagoons, but have not been investigated. Furthermore, with the plans for developing shrimp rearing farms in these coastal lagoons (esteros), contamination by agrochemical residues becomes a matter of much concern for the future of this industry. To provide information on the potential impacts from agriculture and urban development, a screening of the contaminants was carried out in the main lagoons of the Pacific coast. This paper presents the results of the analyses of chlorinated hydrocarbons in lagoon sediments and discusses the ecotoxicological hazard posed by the current levels of persistent pesticide residues to aquatic biota.