In order to determine the 96h-LC , twenty PL were exposed to each of the following 50 nominal concentrations of copper or zinc: 0 (control); 0.5; 1.0; 2.0; 4.0 and 8.0 ppm of 21 21 Cu as CuSO .5H O or Zn as ZnSO .7H O. After 96 h of exposure, LC for 4 2 4 2 50 copper and zinc were determined by means of probit analysis and compared using the respective 95% confidence intervals