Plants and fungi are known as a valuable source of natural medicines used in the treatment of various diseases. Many of them are used to treat human and animal gastrointestinal diseases caused by parasites. The aim of this study was to investigate for the first time the antinematode properties of extracellular low-molecular subfractions (ex-LMS) obtained from the liquid growth medium of idiophasic Cerrena unicolor cultures. The fungal fractions were isolated according to a procedure previously described by Jaszek et al. The in vitro tests were performed using nematodes of the Rhabditis genus. As demonstrated by the results, the total fraction with a molecular weight < 10 kDa (CU-A) and the 0.02–1.5 kDa fraction (CU-B) had nematicidal activity. It was found that the analyzed substances induced movement disturbances caused by the paralysis of the back part of the nematode’s body. The degree of body paralysis was proportional to the increase in the concentration of the tested fractions. Summarizing the obtained results in the context of the available literature data, it seems that C. unicolor may be a good new candidate for research on nematode infections.