The presente study was carried out to determine the short-term effective concentration 50% toxicity (EC50) of methyl parathion, and to evaluate whether Daphnia laevis has the potential to be used as test organism, in medium M4, using potassium dichromate as reference substance. Twenty neonates, aged less than 24h, were separated into four groups, exposed to concentrations of 5x10-6 mg.L-1, 10x10-6 mg.L-1, 20x10-6 mg.L-1, 40x10-6 mg.L-1, 80x10-6 mg.L-1, plus a zero control, for 24h in a static test. Spearman-Karber method was is adoted for estimating of the median effective concentration (EC50). The EC50 of methyl parathion in Daphnia was 19.4 x 10-6 mg.L-1. Thus, Daphnia laevis, in medium M4, has potential to be used as test organism using as reference substance the potassium dichromate.