Abstract: Background and Objectives: Lonchocarpus cyanescensis is a common tropical plant used traditionally by fisher folks to stun and catch fish in Akwa Ibom State, Nigeria. Lonchocarpus cyanescens is a shrub of twining habitat, belonging to the tribe Dalbergiae of the natural order Leguminosae. This study was conducted to determine the bioactive components and assess the acute toxicity of the aqueous extract of the L. cynescens on C. gariepinus fingerlings and anuran tadpoles. Materials and Methods: The 96 h LC50 values of L. cyanescens bark aqueous extracts were determined in the laboratory under static bioassay conditions against C. gariepinus fingerlings and anuran tadpoles. Range finding bioassays were conducted to get the range of concentrations for the definitive bioassays. The range of concentrations of test media for C. gariepinus fingerlings was 0.6-5 mg L–1 while that of anuran tadpoles was 0.6-2 mg L–1. The median lethal concentrations (LC50) were determined using probit analysis. Results: The 96 h LC50 values indicated that L. cynescens was more toxic against C. gariepinus fingerlings than the anuran tadpoles. Unpaired t-test also showed that the test plant was significantly (p<0.05) more toxic against the fingerlings than tadpoles. The mean water quality parameters were within the optimal range requirement for the test species. Conclusion: The study indicated that L. cyanescens exerted piscicidal effects on C. gariepinus fingerlings and anuran tadpoles. The indiscriminate use of this plant to catch fish by local fishermen should be discouraged.