The 96 h LC50 value was found to be 75 ppb. The chronic influence of 8 sublethal concentrations (5, 10, 20, 30.40, 50, 60 and 70 ppb) was studied during the zoeal development. Development was completed in all the test concentrations. The overall survival rate in control and in the lowest two test concentrations (5 and 10 ppb) was 100%. In the other test concentrations, the survival rate decreased with increase in concentration. The survival rate in 20 ppb concentration was 90% and it decreased to 25% in 70 ppb concentration. The intermoult duration in all the four zoeal stages was shorter in control and it increased with increase in test concentration. In the first zoeal stage, the intermoult duration in control was 6.40 days and it increased from 6.61 days in 5 ppb concentration to 7.71 days in 70 ppb concentration. The intermoult duration in control, 5 and 70 ppb concentration for second zoea was 3.60, 3.70 and 5.50 days, for III zoea 3.30, 3.33 and 4.22 days and for IV zoea 3.40, 4.00 and 5.55 days respectively. As the overall survival rate in all the test concentrations was above 10%, the chronic concentrations of the present study fitted in with the definition of Epifanio (1979)