An environmental survey of several rivers of the southern Huasteca area of Mexico revealed high concentrations of manganese (Mn) and the presence of DDT in the sediments and pore water. Therefore, acute (48-h) toxicities of Mn and DDT were assessed both independently and as a combination on 24-h-old neonates of Daphnia magna Strauss and Lecane quadridentata Ehrenberg. Daphnia magna showed high sensitivity to both toxicants, whereas L. quadridentata was highly resistant to DDT and less susceptible to Mn. For D. magna, the Mn and DDT coexposure was significantly more toxic than any of the singly tested compounds. When D. magna was exposed to sediment pore water, no association was found between the Mn content in the samples and the observed toxicity. Preliminary particle analysis of pore water showed different compounds of Mn, which apparently were not in bioavailable form.