Diverse samples were examined at a site of water-bird mortality, caused by Clostridium botulinum type C toxin in southern Moravia (Czechoslovakia). The toxin was detected in high concentrations in mute swan (Cygnus olor) carcasses (≤1 × 10⁶ LD₅₀/g) as well as in necrophagous larvae and pupae of the blow flies Lucilia sericata and Calliphora vomitoria (≤1 × 10⁵ LD₅₀/g) collected from them. It was detected in lower concentrations (≤1 × 10³ LD₅₀/g) in other invertebrates (ptychopterid fly larvae, leeches, sow-bugs) associated with these carcasses, and occasionally in water samples (8 LD₅₀/ml) close to the carrion. The toxin was not detected in the samples of water, mud or invertebrates collected at a distance ≥5 m from the carcasses. The toxin-bearing larvae of L. sericata and C. vomitoria, containing 80,000 LD₅₀/g of type C toxin, were exposed in the mud at the study site for 131 days from November to March. Although the toxin activity decreased 25-fold and 40-fold in the two samples of maggots exposed during this period, it remained very high (≤3,200 LD₅₀/g). Birds ingesting a relatively low number of these toxic larvae (or pupae) in the spring could receive a lethal dose of the toxin.