Two concentrations of the domestic detergent, 2 and 4 mg/l, and one of LAS, 0.8 mg/l, which corresponds roughly to the amount contained in 4 mg of the commercial powder, were tested. Males and females were put into test solutions immediately after having reached sexual maturity, and left there until the first egg sac appeared in females, that is for two to six days. Ovigerous females were then isolated in water without detergent and the number of nauplii produced by the first egg sac was counted. The effect of these detergents depends on the crowding level. When 1 male and 1 female were put in one vessel with 20 ml of water, the presence of the pollutant induced a reduction of the mean number of nauplii, which did not occur in the controls. When 8 males and 8 females were put in the same volume of water, the pollutants induced an increase of the number of nauplii. It is suggested that these apparently paradoxical results depend on two different actions of the detergents. First, they may affect the exchange of gases, so that the following reduced oxygenation may damage the developing eggs. On the other hand, these pollutants appear to remove the mechanism that enables the animals to perceive and to react to crowding. This agrees with a previous hypothesis that the above mechanism might depend on the production by Tisbe of some specific chemical, so far unidentified. Thus, the detergent may interact with such substances and/or damage the chemoreceptors of the animals. In all cases LAS appears to be responsible for the phenomena observed.