Natural substances with great biological potential, i.e., herbicidal, are essential oils (EO). Their use may adversely affect not only weeds but also crop species. Therefore, the study aimed to assess the phytotoxic potential of the EO from the seeds of parsley (Oleum petroselini) against germination and the initial growth of seedlings of two crops: spring wheat (Triticum aestivum L.) and white mustard (Sinapis alba L). The EO was obtained by steam distillation, and its chemical composition was analysed using the GC/MS method. Two series of laboratory experiments were carried out. The oil in water (o/w) solutions with ethyl alcohol (2.0%) were used in five doses: 0.004; 0.007; 0.01; 0.02; 0.03 g liter-1 (w/w). The Petri dishes were lined with filter paper, soaked in the o/w solution. Next, 20 seeds of the tested species were laid out. The dishes were left in the dark at room temperature for seven days. Then the seedlings were counted, and the length of their roots and shoots [mm] were measured. In total, 20 chemical components (98%) were identified in the EO from the parsley seeds, among which α-pinene, β-pinene, and apiol dominated. It was found that spring wheat was more sensitive to the OE than white mustard. The o/w solutions, at the lowest dose already, significantly inhibited the growth of seedlings of both species while not significantly affecting the number of germinated seeds. In conclusion, the EO from parsley seeds has a phytotoxic potential against the tested crops, especially against wheat. Still, the physiological mechanisms of this phenomenon need to be recognized.