The occurrence of microplastics in the aquatic environment has been reported around the world; however, their effects on freshwater oligochaetes are unknown. In this research, we investigated the toxic effects of polyethylene microplastics (MP), size between 40 and 48 μm, on the aquatic worm Allonais inaequalis. We applied the bioassays considering 24 °C as standard temperature and thermal stress of 19 °C and 29 °C, associated with the presence and absence of sediment in short-term and chronic exposures (96 h and 240 h, respectively). MP did not cause significant mortality in short-term exposures and did not affect the reproduction of worms. In addition, when we analyzed whether thermal stress, as well as substrate availability, would have an additional impact on MP toxicity, there were no significant effects. At 29 °C, the individuals reached the highest reproduction rates, whereas at 19 °C the offspring significantly reduced. Moreover, the lack of sediment substantially reduced survival rates after 96 h under 24 °C (p = 0.018). This paper also records for the first time, the ability of microplastic ingestion by a freshwater naidid. Due to its capacity to respond in a short period, adaptation to laboratory cultivation, and representativeness among freshwater aquatic invertebrates, A. inaequalis is presented as a tropical test organism for toxic effect analysis of microplastics, either in conventional exposures or simulated environmental disturbances.