The effect of natural naphtenic acid salts on the rooting of two clones: L-12 (Populus alba) and S6-7 (Populus deltoides) was investigated in the conditions of water culture. Naphtenic acids were produced by alcaline extraction from atmospheric gas oil of the oil "Velebit" and they are purified by chromatography on aluminium oxide. The distinct inhibitory effect on the root and shoot growth in water culture was detected in the concentration of naphtenic acids 10^-4 mol/dm³, while at the concentration 10^-7 mol/dm³ the significant difference form control was not detected. The one hour treatment with 10^-4 mol/dm³ did not significantly influenced the rooting parameters, while the 24-hour treatment raised the number roots on undermost 5 cm of the cutting, as well as the total number of roots comparing to the control. We consider that the positive effect on the primordia activation, as well as inhibitory effect on the root's and shoot's growth indicate the auxinic effect of investigated naphtenic acids.