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The voltammetric behavior of fenbendazole, albendazole, thiabendazole, and mebendazole at a glassy carbon electrode in different aqueous (Britton–Robinson buffer, KOH solutions) and predominantly nonaqueous (acetonitrile:water and methanol:water mixtures) working media is reported. 0.1 M KOH and 70:30 acetonitrile or methanol:water mixtures were selected as most appropriate, the sensitivity achieved for the square wave voltammetric determination of fenbendazole in 0.1 M KOH being much higher than in the predominantly nonaqueous media. Flow-injection with amperometric detection of the benzimidazoles was evaluated in the three working media. The electrode responded rapidly and the response was constant over sets of 30 measurements at the selected potentials in all cases, RSD values ranging between 1.6% and 4.4%. No significant electrode surface fouling was observed under the hydrodynamic conditions. Detection limits in the 1.0 × 10−8–1.0 × 10−7 M concentration range were achieved. As an application, fenbendazole was determined by flow injection with amperometric detection in spiked feed samples at the 30 and 180 µg g−1 concentration levels, which correspond to the lower and upper concentration for fenbendazole administration to treated poultry. Recoveries of 97 ± 5% and 94 ± 8% were obtained. [ABSTRACT FROM AUTHOR]