Routine toxicity screening of substances using rodents can be time consuming and expensive. For these reasons various alternative methods have been developed, but many lack standardization and acceptance. It was hypothesized that a satisfactory classification of mammalian toxicants could be obtained using a simple mosquito (Aedes aegypti) screening method. To this end, median lethal concentration (LC50) values were determined for a wide variety of mammalian toxicants representing a range of toxicity classes. Each substance was assayed in aqueous solution by incubation for different times, up to 24 hr, using at least 30 individual larvae and six different concentrations, for valid statistical analysis. With few exceptions, the resultant toxicity classes were the same as those determined using mammalian models. This classification was validated further by the very high correlation (r = 0.97) determined between the mosquito LC50 values and the rat LD50 values. These results suggest that this mosquito method can be used to rapidly screen compounds of potential mammalian toxicity.