The objective of this study was to investigate the uptake of deltamethrin, an insecticide, by Daphnia magna neonates by SIMS and to compare these findings with results based on established toxicity tests. Young daphnids (aged <24 h) were exposed to 0, 50 and 200 microg L(-1) (ppb) deltamethrin. Mobile, immobile and dead animals were enumerated after 24 and 48 h following OECD 202 [OECD 202, 2004. Daphnia sp., acute immobilisation test, guideline for testing of chemicals] guidelines. The animals were embedded in epoxy resin, cut into semi-thin sections (500 nm) and placed on silicon supporters. NanoSIMS 50 (Cameca) images were made from tissues of the intestine for carbon, nitrogen (measured as CN), phosphorus and bromine. To distinguish between relative concentrations of bromine in the guts from different exposure concentrations of deltamethrin, a carbon normalization method was carried out. Both deltamethrin concentrations and time showed a significant effect on immobilization and mortality of the daphnids (P<0.0001). Bromine from deltamethrin could be visualized by NanoSIMS in all exposed gut tissues (gut wall, microvilli layer, perithropic membrane). Highest deltamethrin concentrations following (12)C normalization were found in animals exposed to 200 microg L(-1) deltamethrin, followed by 50 microg L(-1) and the control. NanoSIMS 50 was successfully used as a supplemental technique for elucidating the relation between the uptake and localization of deltamethrin and its toxicity to D. magna. These results highlight the potential usefulness of NanoSIMS to detect marker elements of xenobiotic compounds within exposed organisms, to compare relative exposure concentrations, and to locate these compounds at their original tissue location.