Chlorpyrifos, O,O-diethyl-O-(3,5,6-trichloro-2-pyridyl) phosphorothioate, is one of the most widely used organophosphorous pesticides worldwide due its high efficacy (Carvalho et al. 1992; Liu et al. 2001, Meikle and Youngson 1978). In Kenya, Dursban formulations are mainly used for protection of horticultural fruits and vegetables (Lalah 1994), Because of its low water solubility (0.4 mg/L) and high hydrophobicity (log Kow of 5.27), it is believed that chlorpyrifos would be able to partition easily onto aquatic sediments and macrophytes where it can pose dangers to benthic organisms (Ronday et al. 1998). It is also quite a volatile toxicant due to its low vapour pressure (2.5 mPa at 25 °C) and low air-to-water partition coefficient (8.9 10"4 at 25 °C) and its residues have been detected in air and rainwater samples (Liu et al. 2001).Increasing use of chlorpyrifos also causes a lot of anxiety to environmentalists and toxicologists because it is toxic to both humans and wildlife. As an irreversible inhibitor of acetylcholinesterase, it can cause impairment in mammalian brain cell development (Lund et al. 2000; Jeanty et al 2001; Jett et al 1999; Slotkin et al. 2001). Widespread use of this compound is therefore considered to be of great danger particularly to pregnant women and children Some of its reported toxicities to aquatic organisms include 96 hr LCNo of 0.13 |.ig/L and 96 hr LC?o of 0.035 Ug/L in adult Neomysis integer and Americamysis bahia, respectively (Roast et al. 1999). Although its toxicity in mammalian and aquatic organisms has been well documented, its fate and effects on aquatic ecosystems in tropical conditions where it is expected to degrade and dissipate faster (Carvalho et al 1992) are little known. In a laboratory model aquarium simulating a tropical marine environment, we studied the persistence and accumulation of HC-chlorpyrifos in sediment, fish and oysters. The results obtained from these studies are reported in this paper.