A major technical difficulty associated with testing the toxicity to aquatic organisms of chemicals having low water solubility is the production of a stock solution of the chemical without the use of exogenous solubilizers or carriers. For highly toxic hydrophobic chemicals this problem has been approached by passing water through a column of chemical adsorbed to inert supports (CHADWICK & KIIGIMAGI 1968) o While this technique is somewhat effective, channelization of water in the column may lead to variation in the concentration of toxicant produced during long term exposure studies. VEITH & COMSTOCK (1975) reduced this variation by continuously recirculating the toxicant solution through the bed of substrate with a submersible pump. For many hydrophobic chemicals of moderate or low toxicity, the difficulties of producing a satisfactory stock solution are compounded by the fact that water concentrations of the chemical that are acutely toxic to fish may be near their limits of solubility~ Toxicity testing of these chemicals through continuous flow bioassay methods thus requires large volumes of stock solution of relatively high concentration. In addition, many toxicants are liquids at test temperatures and not amenable to application from a substrate bed.