rtemia salina nauplii are widely used in toxicity assays of synthetic and natural products. Microassays are particularly useful because they allow testing of several products, serially diluted, in a sole microplate. Despite this, following the original method, we have observed systematically about 30% mortality in nontreated controls. Thus, we introduced several modifications that improved the viability percentage and motility of nauplii. To standardize the present method, we used a dilution series (100.00–0.19 µg/ml) of potassium dichromate (K2Cr2O7) and calculated the concentration that killed 50% of the nauplii (LC50) by PROBIT analysis. The above modifications allowed us to obtain nauplii cultures that have systematically 100% viability and good motility until 44 h after hatching. The LC50 of K2Cr2O7 (12.60 µg/ml) was similar to that previously reported (12.50 µg/ml). We propose this modified microassay to determine the toxicity of synthetic and natural products, as an alternative that allows the biological material to remain in better condition throughout the entire assay, and good reproducibility of results.