IntroductionPassive samplers, such as semi-permeable membrane devices (SPMDs), are rapidly being adopted for environmental monitoring of hydrophobic organic compounds. SPMDs can be deployed in aquatic environments for a number of different applications including determining the presence, source, and fate of contaminants; delineating contaminant fate in different environmental compartments; estimating time-weighted average contaminant concentrations; predicting organism exposure; and biomimetic extraction for toxicity assessment of aqueous contaminants.1Since the SPMD design was first introduced in 1990,2SPMDs have proven to be extremely efficient for monitoring hydrophobic contaminants in the environment, and over 300 studies have been published.3However, no studies are known to have been conducted in tropical regions or in tropical streams.Pesticide use in Central America is a concern, and several environmental and human poisonings have been documented.4–6However, few studies regarding environmental distribution, toxic effects on aquatic organisms, or general impact on ecosystems have been undertaken in tropical areas.5,6Similarly, no studies have examined the effect of biofouling on deployed SPMDs in tropical streams, even though SPMDs are known to experience heavy biofouling especially during extended exposures in warm or tropical surface waters.1This study was conducted to increase understanding of the interaction among biofouling, deployment time, and stream flow on pesticide sequestration in stream-deployed SPMDs. This first use of passive samplers, such as SPMDs, in Central America is part a larger study to detect and monitor pesticide presence, fate, and biological effects in the extensive agricultural region of the Sixaola River watershed in southeastern Costa Rica.7