In Wolffia arrhiza (Lemnaceae) growing on municipal tap water (rich in mineral but poor in organic components) and thus being exclusively photoautotrophic, IAA (3-indolilacetic acid) and SA (salicylic acid) strongly stimulated action on the content of chlorophylls a and b and carotenoids (especially b-carotene and lutein + zeaxanthin). On the other hand, the chemical analogues of IAA, i.e. PAA (phenylacetic acid) and NAA (a-naphtylacetic acid), had a generally inhibitory effect on chlorophyll and carotenoid contents. In Wolffia arrhiza growing on raw waste water and a suspension of activated sludge from a sewage treatment plant (rich in organic substances) characterized by mixotrophism (that is photo- and heterotrophism) PAA had the highest stimulative action on the chlorophyll a and b content, SA negligible, whereas NAA had an inhibitory effect. IAA had a slight stimulative effect on raw sewage but inhibitory on activated sludge. Also, the greatest stimulative effect on carotenoids content was exerted by PAA; SA had a slight stimulative effect while IAA and, to a greater extent NAA had a clearly inhibitory influence.