A Ceriodaphnia algal uptake suppression test (CAUST) was developed for short-term toxicity screening. The test is based on the feeding behavior of C. dubia and requires as little as I hour contact time between C. dubia neonates and toxicant. Before the test, neonate digestive tracts were cleared by feeding yeast to the daphnids. Neonates were then exposed to toxicant, followed by addition of Scenedesmus subspicatus to the bioassay vessels. Daphnids were examined under bright-field microscopy for presence of algae (indicated by a green-colored digestive tract) or absence of algae. Uptake indicated no toxic effect, whereas absence of uptake indicated toxic inhibition. Median effective concentrations (ECso values) were computed based on results over a range of dosed toxicant concentrations. Results for cadmium, zinc, copper, pentachlorophenol, phenol, cyanide, and selected pesticides showed that CAUST was similar in sensitivity to the standard 48-hour C. dubia acute bioassay and was as sensitive as or more sensitive than the I-hour LQ. test and 30-minute Microtox. Water Environ.